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[PATCH RFC] fixup! virtio: convert to use DMA api

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  [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-18 13:50 +0200
    Re: [PATCH RFC] fixup! virtio: convert to use DMA api David Woodhouse <dwmw2@infradead.org> - 2016-04-18 14:00 +0200
      Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-18 15:20 +0200
        Re: [PATCH RFC] fixup! virtio: convert to use DMA api David Woodhouse <dwmw2@infradead.org> - 2016-04-18 16:10 +0200
          Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-18 16:30 +0200
            Re: [PATCH RFC] fixup! virtio: convert to use DMA api David Woodhouse <dwmw2@infradead.org> - 2016-04-18 17:30 +0200
              Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-18 17:40 +0200
                Re: [PATCH RFC] fixup! virtio: convert to use DMA api David Woodhouse <dwmw2@infradead.org> - 2016-04-18 18:00 +0200
                  Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-18 18:30 +0200
                    Re: [PATCH RFC] fixup! virtio: convert to use DMA api David Woodhouse <dwmw2@infradead.org> - 2016-04-18 20:30 +0200
                      Re: [PATCH RFC] fixup! virtio: convert to use DMA api Andy Lutomirski <luto@amacapital.net> - 2016-04-18 21:30 +0200
                        Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-19 12:30 +0200
                          Re: [PATCH RFC] fixup! virtio: convert to use DMA api Andy Lutomirski <luto@amacapital.net> - 2016-04-19 18:10 +0200
                            Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-19 18:10 +0200
                              Re: [PATCH RFC] fixup! virtio: convert to use DMA api Andy Lutomirski <luto@amacapital.net> - 2016-04-19 18:20 +0200
                                Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-19 18:30 +0200
                                  Re: [PATCH RFC] fixup! virtio: convert to use DMA api David Woodhouse <dwmw2@infradead.org> - 2016-04-19 19:40 +0200
                                    Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-19 20:00 +0200
                                      Re: [PATCH RFC] fixup! virtio: convert to use DMA api Andy Lutomirski <luto@amacapital.net> - 2016-04-19 20:10 +0200
                                        Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-19 22:20 +0200
                                          Re: [PATCH RFC] fixup! virtio: convert to use DMA api Andy Lutomirski <luto@amacapital.net> - 2016-04-19 22:30 +0200
                                            Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-19 23:00 +0200
                                              Re: [PATCH RFC] fixup! virtio: convert to use DMA api Andy Lutomirski <luto@amacapital.net> - 2016-04-19 23:10 +0200
                                                Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-20 15:20 +0200
                                                  Re: [PATCH RFC] fixup! virtio: convert to use DMA api Andy Lutomirski <luto@amacapital.net> - 2016-04-20 17:50 +0200
                      Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-19 11:20 +0200
                        Re: [PATCH RFC] fixup! virtio: convert to use DMA api Alex Williamson <alex.williamson@redhat.com> - 2016-04-19 16:50 +0200
                        Re: [PATCH RFC] fixup! virtio: convert to use DMA api Andy Lutomirski <luto@amacapital.net> - 2016-04-19 18:10 +0200
                          Re: [PATCH RFC] fixup! virtio: convert to use DMA api "Michael S. Tsirkin" <mst@redhat.com> - 2016-04-19 18:10 +0200

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#1381629 — [PATCH RFC] fixup! virtio: convert to use DMA api

This adds a flag to enable/disable bypassing the IOMMU by
virtio devices.

This is on top of patch
http://article.gmane.org/gmane.comp.emulators.qemu/403467
virtio: convert to use DMA api

Tested with patchset
http://article.gmane.org/gmane.linux.kernel.virtualization/27545
virtio-pci: iommu support

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

---
 include/hw/virtio/virtio-access.h              | 3 ++-
 include/hw/virtio/virtio.h                     | 6 +++++-
 include/standard-headers/linux/virtio_config.h | 8 ++++++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/hw/virtio/virtio-access.h b/include/hw/virtio/virtio-access.h
index 967cc75..bb6f34e 100644
--- a/include/hw/virtio/virtio-access.h
+++ b/include/hw/virtio/virtio-access.h
@@ -23,7 +23,8 @@ static inline AddressSpace *virtio_get_dma_as(VirtIODevice *vdev)
     BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
 
-    if (k->get_dma_as) {
+    if ((vdev->host_features & (0x1ULL << VIRTIO_F_IOMMU_PLATFORM)) &&
+        k->get_dma_as) {
         return k->get_dma_as(qbus->parent);
     }
     return &address_space_memory;
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index b12faa9..34d3041 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -228,7 +228,11 @@ typedef struct VirtIORNGConf VirtIORNGConf;
     DEFINE_PROP_BIT64("notify_on_empty", _state, _field,  \
                       VIRTIO_F_NOTIFY_ON_EMPTY, true), \
     DEFINE_PROP_BIT64("any_layout", _state, _field, \
-                      VIRTIO_F_ANY_LAYOUT, true)
+                      VIRTIO_F_ANY_LAYOUT, true), \
+    DEFINE_PROP_BIT64("iommu_passthrough", _state, _field, \
+                      VIRTIO_F_IOMMU_PASSTHROUGH, false), \
+    DEFINE_PROP_BIT64("iommu_platform", _state, _field, \
+                      VIRTIO_F_IOMMU_PLATFORM, false)
 
 hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n);
 hwaddr virtio_queue_get_avail_addr(VirtIODevice *vdev, int n);
diff --git a/include/standard-headers/linux/virtio_config.h b/include/standard-headers/linux/virtio_config.h
index bcc445b..5564dab 100644
--- a/include/standard-headers/linux/virtio_config.h
+++ b/include/standard-headers/linux/virtio_config.h
@@ -61,4 +61,12 @@
 /* v1.0 compliant. */
 #define VIRTIO_F_VERSION_1		32
 
+/* Request IOMMU passthrough (if available)
+ * Without VIRTIO_F_IOMMU_PLATFORM: bypass the IOMMU even if enabled.
+ * With VIRTIO_F_IOMMU_PLATFORM: suggest disabling IOMMU.
+ */
+#define VIRTIO_F_IOMMU_PASSTHROUGH	33
+
+/* Do not bypass the IOMMU (if configured) */
+#define VIRTIO_F_IOMMU_PLATFORM		34
 #endif /* _LINUX_VIRTIO_CONFIG_H */
-- 
MST

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#1381631

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On Mon, 2016-04-18 at 14:47 +0300, Michael S. Tsirkin wrote:
> This adds a flag to enable/disable bypassing the IOMMU by
> virtio devices.

I'm still deeply unhappy with having this kind of hack in the virtio
code at all, as you know. Drivers should just use the DMA API and if
the *platform* wants to make it a no-op for a specific device, then it
can.

Remember, this isn't just virtio either. Don't we have *precisely* the
same issue with assigned PCI devices on a system with an emulated Intel
IOMMU? The assigned PCI devices aren't covered by the emulated IOMMU,
and the platform needs to know to bypass *those* too.

Now, we've had this conversation, and we accepted the hack in virtio
for now until the platforms (especially SPARC and Power IIRC) can get
their act together and make their DMA API implementations not broken.

But now you're adding this hack to the public API where we have to
support it for ever. Please, can't we avoid that?

-- 
dwmw2

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#1381691

On Mon, Apr 18, 2016 at 07:58:37AM -0400, David Woodhouse wrote:
> On Mon, 2016-04-18 at 14:47 +0300, Michael S. Tsirkin wrote:
> > This adds a flag to enable/disable bypassing the IOMMU by
> > virtio devices.
> 
> I'm still deeply unhappy with having this kind of hack in the virtio
> code at all, as you know. Drivers should just use the DMA API and if
> the *platform* wants to make it a no-op for a specific device, then it
> can.
> 
> Remember, this isn't just virtio either. Don't we have *precisely* the
> same issue with assigned PCI devices on a system with an emulated Intel
> IOMMU? The assigned PCI devices aren't covered by the emulated IOMMU,
> and the platform needs to know to bypass *those* too.
> 
> Now, we've had this conversation, and we accepted the hack in virtio
> for now until the platforms (especially SPARC and Power IIRC) can get
> their act together and make their DMA API implementations not broken.
> 
> But now you're adding this hack to the public API where we have to
> support it for ever. Please, can't we avoid that?

I'm not sure I understand the issue.  The public API is not about how
the driver works.  It doesn't say "don't use DMA API" anywhere, does it?
It's about telling device whether to obey the IOMMU and
about discovering whether a device is in fact under the IOMMU.

Once DMA API allows bypassing IOMMU per device we'll be
able to drop the ugly hack from virtio drivers, simply keying it
off the given flag.


> -- 
> dwmw2
> 
> 

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#1381732

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On Mon, 2016-04-18 at 16:12 +0300, Michael S. Tsirkin wrote:
> I'm not sure I understand the issue.  The public API is not about how
> the driver works.  It doesn't say "don't use DMA API" anywhere, does it?
> It's about telling device whether to obey the IOMMU and
> about discovering whether a device is in fact under the IOMMU.

Apologies, I was wrongly reading this as a kernel patch.

After a brief struggle with "telling device whether to obey the IOMMU",
which is obviously completely impossible from the guest kernel, I
realise my mistake :)

So... on x86 how does this get reflected in the DMAR tables that the
guest BIOS presents to the guest kernel, so that the guest kernel
*knows* which devices are behind which IOMMU?

(And are you fixing the case of assigned PCI devices, which aren't
behind any IOMMU, at the same time as you answer that? :)

-- 
dwmw2

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#1381755

On Mon, Apr 18, 2016 at 10:03:52AM -0400, David Woodhouse wrote:
> On Mon, 2016-04-18 at 16:12 +0300, Michael S. Tsirkin wrote:
> > I'm not sure I understand the issue.  The public API is not about how
> > the driver works.  It doesn't say "don't use DMA API" anywhere, does it?
> > It's about telling device whether to obey the IOMMU and
> > about discovering whether a device is in fact under the IOMMU.
> 
> Apologies, I was wrongly reading this as a kernel patch.
> 
> After a brief struggle with "telling device whether to obey the IOMMU",
> which is obviously completely impossible from the guest kernel, I
> realise my mistake :)
> 
> So... on x86 how does this get reflected in the DMAR tables that the
> guest BIOS presents to the guest kernel, so that the guest kernel
> *knows* which devices are behind which IOMMU?

This patch doesn't change DMAR tables, it creates a way for virtio
device to tell guest "I obey what DMAR tables tell you, you can stop
doing hacks".

And as PPC guys seem adamant that platform tools there are no good for
that purpose, there's another bit that says "ignore what platform tells
you, I'm not a real device - I'm part of hypervisor and I bypass the
IOMMU".


> (And are you fixing the case of assigned PCI devices, which aren't
> behind any IOMMU, at the same time as you answer that? :)

No - Aviv B.D. has patches on list to fix that.

-- 
MST

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#1381829

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On Mon, 2016-04-18 at 17:23 +0300, Michael S. Tsirkin wrote:
> 
> This patch doesn't change DMAR tables, it creates a way for virtio
> device to tell guest "I obey what DMAR tables tell you, you can stop
> doing hacks".
> 
> And as PPC guys seem adamant that platform tools there are no good for
> that purpose, there's another bit that says "ignore what platform tells
> you, I'm not a real device - I'm part of hypervisor and I bypass the
> IOMMU".

...

+/* Request IOMMU passthrough (if available)
+ * Without VIRTIO_F_IOMMU_PLATFORM: bypass the IOMMU even if enabled.
+ * With VIRTIO_F_IOMMU_PLATFORM: suggest disabling IOMMU.
+ */
+#define VIRTIO_F_IOMMU_PASSTHROUGH     33
+
+/* Do not bypass the IOMMU (if configured) */
+#define VIRTIO_F_IOMMU_PLATFORM                34

OK... let's see if I can reconcile those descriptions coherently.

Setting (only) VIRTIO_F_IOMMU_PASSTHROUGH indicates to the guest that
its own operating system's IOMMU code is expected to be broken, and
that the virtio driver should eschew the DMA API? And that the guest OS
cannot further assign the affected device to any of *its* nested
guests? Not that the broken IOMMU code in said guest OS will know the
latter, of course.

With VIRTIO_F_IOMMU_PLATFORM set, VIRTIO_F_IOMMU_PASSTHROUGH is just a
*hint*, suggesting that the guest OS should *request* a passthrough
mapping from the IOMMU? Via a driver←→IOMMU API which doesn't yet exist
in Linux, since we only have 'iommu=pt' on the command line for that?

And having *neither* of those bits sets is the status quo, which means
that your OS code might well be broken and need you to eschew the DMA
API, but maybe not.


-- 
dwmw2

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#1381839

On Mon, Apr 18, 2016 at 11:22:03AM -0400, David Woodhouse wrote:
> On Mon, 2016-04-18 at 17:23 +0300, Michael S. Tsirkin wrote:
> > 
> > This patch doesn't change DMAR tables, it creates a way for virtio
> > device to tell guest "I obey what DMAR tables tell you, you can stop
> > doing hacks".
> > 
> > And as PPC guys seem adamant that platform tools there are no good for
> > that purpose, there's another bit that says "ignore what platform tells
> > you, I'm not a real device - I'm part of hypervisor and I bypass the
> > IOMMU".
> 
> ...
> 
> +/* Request IOMMU passthrough (if available)
> + * Without VIRTIO_F_IOMMU_PLATFORM: bypass the IOMMU even if enabled.
> + * With VIRTIO_F_IOMMU_PLATFORM: suggest disabling IOMMU.
> + */
> +#define VIRTIO_F_IOMMU_PASSTHROUGH     33
> +
> +/* Do not bypass the IOMMU (if configured) */
> +#define VIRTIO_F_IOMMU_PLATFORM                34
> 
> OK... let's see if I can reconcile those descriptions coherently.
> 
> Setting (only) VIRTIO_F_IOMMU_PASSTHROUGH indicates to the guest that
> its own operating system's IOMMU code is expected to be broken, and
> that the virtio driver should eschew the DMA API?

No - it tells guest that e.g. the ACPI tables (or whatever the
equivalent is) do not match reality with respect to this device
since IOMMU is ignored by hypervisor.
Hypervisor has no idea what does guest IOMMU code do - hopefully
it is not actually broken.

> And that the guest OS
> cannot further assign the affected device to any of *its* nested
> guests? Not that the broken IOMMU code in said guest OS will know the
> latter, of course.
> 
> With VIRTIO_F_IOMMU_PLATFORM set, VIRTIO_F_IOMMU_PASSTHROUGH is just a
> *hint*, suggesting that the guest OS should *request* a passthrough
> mapping from the IOMMU?

Right. But it'll work correctly if you don't.

> Via a driver←→IOMMU API which doesn't yet exist
> in Linux, since we only have 'iommu=pt' on the command line for that?
> 
> And having *neither* of those bits sets is the status quo, which means
> that your OS code might well be broken and need you to eschew the DMA
> API, but maybe not.


The status quo is that that the IOMMU might well be bypassed
and then you need to program physical addresses into the device,
but maybe not. If DMA API does not give you physical addresses, you
need to bypass it, but hypervisor does not know or care.


> 
> -- 
> dwmw2
> 
> 

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#1381847

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On Mon, 2016-04-18 at 18:30 +0300, Michael S. Tsirkin wrote:
> 
> > Setting (only) VIRTIO_F_IOMMU_PASSTHROUGH indicates to the guest that
> > its own operating system's IOMMU code is expected to be broken, and
> > that the virtio driver should eschew the DMA API?
> 
> No - it tells guest that e.g. the ACPI tables (or whatever the
> equivalent is) do not match reality with respect to this device
> since IOMMU is ignored by hypervisor.
> Hypervisor has no idea what does guest IOMMU code do - hopefully
> it is not actually broken.

OK, that makes sense — thanks.

So where the platform *does* have a way to coherently tell the guest
that some devices are behind and IOMMU and some aren't, we should never
see VIRTIO_F_IOMMU_PASSTHROUGH && !VIRTIO_F_IOMMU_PLATFORM. (Except
perhaps temporarily on x86 until we *do* fix the DMAR tables to tell
the truth; qv.)

This should *only* be a crutch for platforms which cannot properly
convey that information from the hypervisor to the guest. It should be
clearly documented "thou shalt not use this unless you've first
attempted to fix the broken platform to get it right for itself".

And if we look at it as such... does it make more sense for this to be
a more *generic* qemu←→guest interface? That way the software hacks can
live in the OS IOMMU code where they belong, and prevent assignment to
nested guests for example. And can cover cases like assigned PCI
devices in existing qemu/x86 which need the same treatment.

Put another way: if we're going to add code to the guest OS to look at
this information, why can't we add that code in the guest's IOMMU
support instead, to look at an out-of-band qemu-specific "ignore IOMMU
for these devices" list instead?

> The status quo is that that the IOMMU might well be bypassed
> and then you need to program physical addresses into the device,
> but maybe not. If DMA API does not give you physical addresses, you
> need to bypass it, but hypervisor does not know or care.

Right. The status quo is that qemu doesn't provide correct information
about IOMMU topology to guests, and they have to have heuristics to
work out whether to eschew the IOMMU for a given device or not. This is
true for virtio and assigned PCI devices alike.

Furthermore, some platforms don't *have* a standard way for qemu to
'tell the truth' to the guests, and that's where the real fun comes in.
But still, I'd like to see a generic solution for that lack instead of
a virtio-specific hack.

-- 
dwmw2

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#1381876

On Mon, Apr 18, 2016 at 11:51:41AM -0400, David Woodhouse wrote:
> On Mon, 2016-04-18 at 18:30 +0300, Michael S. Tsirkin wrote:
> > 
> > > Setting (only) VIRTIO_F_IOMMU_PASSTHROUGH indicates to the guest that
> > > its own operating system's IOMMU code is expected to be broken, and
> > > that the virtio driver should eschew the DMA API?
> > 
> > No - it tells guest that e.g. the ACPI tables (or whatever the
> > equivalent is) do not match reality with respect to this device
> > since IOMMU is ignored by hypervisor.
> > Hypervisor has no idea what does guest IOMMU code do - hopefully
> > it is not actually broken.
> 
> OK, that makes sense — thanks.
> 
> So where the platform *does* have a way to coherently tell the guest
> that some devices are behind and IOMMU and some aren't, we should never
> see VIRTIO_F_IOMMU_PASSTHROUGH && !VIRTIO_F_IOMMU_PLATFORM. (Except
> perhaps temporarily on x86 until we *do* fix the DMAR tables to tell
> the truth; qv.)
> 
> This should *only* be a crutch for platforms which cannot properly
> convey that information from the hypervisor to the guest. It should be
> clearly documented "thou shalt not use this unless you've first
> attempted to fix the broken platform to get it right for itself".
> 
> And if we look at it as such... does it make more sense for this to be
> a more *generic* qemu←→guest interface? That way the software hacks can
> live in the OS IOMMU code where they belong, and prevent assignment to
> nested guests for example. And can cover cases like assigned PCI
> devices in existing qemu/x86 which need the same treatment.
>
> Put another way: if we're going to add code to the guest OS to look at
> this information, why can't we add that code in the guest's IOMMU
> support instead, to look at an out-of-band qemu-specific "ignore IOMMU
> for these devices" list instead?

I balk at adding more hacks to a broken system. My goals are
merely to
- make things work correctly with an IOMMU and new guests,
  so people can use userspace drivers with virtio devices
- prevent security risks when guest kernel mistakenly thinks
  it's protected by an IOMMU, but in fact isn't
- avoid breaking any working configurations

Looking at guest code, it looks like virtio was always
bypassing the IOMMU even if configured, but no other
guest driver did.

This makes me think the problem where guest drivers
ignore the IOMMU is virtio specific
and so a virtio specific solution seems cleaner.

The problem for assigned devices is IMHO different: they bypass
the guest IOMMU too but no guest driver knows about this,
so guests do not work. Seems cleaner to fix QEMU to make
existing guests work.


> > The status quo is that that the IOMMU might well be bypassed
> > and then you need to program physical addresses into the device,
> > but maybe not. If DMA API does not give you physical addresses, you
> > need to bypass it, but hypervisor does not know or care.
> 
> Right. The status quo is that qemu doesn't provide correct information
> about IOMMU topology to guests, and they have to have heuristics to
> work out whether to eschew the IOMMU for a given device or not. This is
> true for virtio and assigned PCI devices alike.

True but I think we should fix QEMU to shadow IOMMU
page tables for assigned devices. This seems rather
possible with VT-D, and there are patches already on list.

It looks like this will fix all legacy guests which is
much nicer than what you suggest which will only help new guests.

> Furthermore, some platforms don't *have* a standard way for qemu to
> 'tell the truth' to the guests, and that's where the real fun comes in.
> But still, I'd like to see a generic solution for that lack instead of
> a virtio-specific hack.

But the issue is not just these holes.  E.g. with VT-D it is only easy
to emulate because there's a "caching mode" hook. It is fundamentally
paravirtualization.  So a completely generic solution would be a
paravirtualized IOMMU interface, replacing VT-D for VMs. It might be
justified if many platforms have hard to emulate interfaces.



> -- 
> dwmw2
> 
> 

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#1381967

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On Mon, 2016-04-18 at 19:27 +0300, Michael S. Tsirkin wrote:
> I balk at adding more hacks to a broken system. My goals are
> merely to
> - make things work correctly with an IOMMU and new guests,
>   so people can use userspace drivers with virtio devices
> - prevent security risks when guest kernel mistakenly thinks
>   it's protected by an IOMMU, but in fact isn't
> - avoid breaking any working configurations

AFAICT the VIRTIO_F_IOMMU_PASSTHROUGH thing seems orthogonal to this.
That's just an optimisation, for telling an OS "you don't really need
to bother with the IOMMU, even though you it works".

There are two main reasons why an operating system might want to use
the IOMMU via the DMA API for native drivers: 
 - To protect against driver bugs triggering rogue DMA.
 - To protect against hardware (or firmware) bugs.

With virtio, the first reason still exists. But the second is moot
because the device is part of the hypervisor and if the hypervisor is
untrustworthy then you're screwed anyway... but then again, in SoC
devices you could replace 'hypervisor' with 'chip' and the same is
true, isn't it? Is there *really* anything virtio-specific here?

Sure, I want my *external* network device on a PCIe card with software-
loadable firmware to be behind an IOMMU because I don't trust it as far
as I can throw it. But for on-SoC devices surely the situation is
*just* the same as devices provided by a hypervisor?

And some people want that external network device to use passthrough
anyway, for performance reasons.

On the whole, there are *plenty* of reasons why we might want to have a
passthrough mapping on a per-device basis, and I really struggle to
find justification for having this 'hint' in a virtio-specific way.

And it's complicating the discussion of the *actual* fix we're looking
at.

> Looking at guest code, it looks like virtio was always
> bypassing the IOMMU even if configured, but no other
> guest driver did.
> 
> This makes me think the problem where guest drivers
> ignore the IOMMU is virtio specific
> and so a virtio specific solution seems cleaner.
> 
> The problem for assigned devices is IMHO different: they bypass
> the guest IOMMU too but no guest driver knows about this,
> so guests do not work. Seems cleaner to fix QEMU to make
> existing guests work.

I certainly agree that it's better to fix QEMU. Whether devices are
behind an IOMMU or not, the DMAR tables we expose to a guest should
tell the truth.

Part of the issue here is virtio-specific; part isn't.

Basically, we have a conjunction of two separate bugs which happened to
work (for virtio) — the IOMMU support in QEMU wasn't working for virtio
(and assigned) devices even though it theoretically *should* have been,
and the virtio drivers weren't using the DMA API as they theoretically
should have been.

So there were corner cases like assigned PCI devices, and real hardware
implementations of virtio stuff (and perhaps virtio devices being
assigned to nested guests) which didn't work. But for the *common* use
case, one bug cancelled out the other.

Now we want to fix both bugs, and of course that involves carefully
coordinating both fixes.

I *like* your idea of a flag from the hypervisor which essentially says
"trust me, I'm telling the truth now".

But don't think that wants to be virtio-specific, because we actually
want it to cover *all* the corner cases, not just the common case which
*happened* to work before due to the alignment of the two previous
bugs.

An updated guest OS can look for this flag (in its generic IOMMU code)
and can apply a heuristic of its own to work out which devices *aren't*
behind the IOMMU, if the flag isn't present. And it can get that right
even for assigned devices, so that new kernels can run happily even on
today's QEMU instances. And the virtio driver in new kernels should
just use the DMA API and expect it to work. Just as the various drivers
for assigned PCI devices do.

The other interesting case for compatibility is old kernels running in
a new QEMU. And for that case, things are likely to break if you
suddenly start putting the virtio devices behind an IOMMU. There's
nothing you can do on ARM and Power to stop that breakage, since they
don't *have* a way to tell legacy guests that certain devices aren't
translated. So I suspect you probably can't enable virtio-behind-IOMMU
in QEMU *ever* for those platforms as the default behaviour.

For x86, you *can* enable virtio-behind-IOMMU if your DMAR tables tell
the truth, and even legacy kernels ought to cope with that.
FSVO 'ought to' where I suspect some of them will actually crash with a
NULL pointer dereference if there's no "catch-all" DMAR unit in the
tables, which puts it back into the same camp as ARM and Power.


> True but I think we should fix QEMU to shadow IOMMU
> page tables for assigned devices. This seems rather
> possible with VT-D, and there are patches already on list.
> 
> It looks like this will fix all legacy guests which is
> much nicer than what you suggest which will only help new guests.

Yes, we should do that. And in the short term we should at *least* fix
the DMAR tables to tell the truth.

> > 
> > Furthermore, some platforms don't *have* a standard way for qemu to
> > 'tell the truth' to the guests, and that's where the real fun comes in.
> > But still, I'd like to see a generic solution for that lack instead of
> > a virtio-specific hack.
> But the issue is not just these holes.  E.g. with VT-D it is only easy
> to emulate because there's a "caching mode" hook. It is fundamentally
> paravirtualization.  So a completely generic solution would be a
> paravirtualized IOMMU interface, replacing VT-D for VMs. It might be
> justified if many platforms have hard to emulate interfaces.

Hm, I'm not sure I understand the point here.

Either there is a way for the hypervisor to expose an IOMMU to a guest
(be it full hardware virt, or paravirt). Or there isn't.

If there is, it doesn't matter *how* it's done. And if there isn't, the
whole discussion is moot anyway.

-- 
dwmw2

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#1381992

On Mon, Apr 18, 2016 at 11:29 AM, David Woodhouse <dwmw2@infradead.org> wrote:
> For x86, you *can* enable virtio-behind-IOMMU if your DMAR tables tell
> the truth, and even legacy kernels ought to cope with that.
> FSVO 'ought to' where I suspect some of them will actually crash with a
> NULL pointer dereference if there's no "catch-all" DMAR unit in the
> tables, which puts it back into the same camp as ARM and Power.

I think x86 may get a bit of a free pass here.  AFAIK the QEMU IOMMU
implementation on x86 has always been "experimental", so it just might
be okay to change it in a way that causes some older kernels to OOPS.

--Andy

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#1382375

On Mon, Apr 18, 2016 at 12:24:15PM -0700, Andy Lutomirski wrote:
> On Mon, Apr 18, 2016 at 11:29 AM, David Woodhouse <dwmw2@infradead.org> wrote:
> > For x86, you *can* enable virtio-behind-IOMMU if your DMAR tables tell
> > the truth, and even legacy kernels ought to cope with that.
> > FSVO 'ought to' where I suspect some of them will actually crash with a
> > NULL pointer dereference if there's no "catch-all" DMAR unit in the
> > tables, which puts it back into the same camp as ARM and Power.
> 
> I think x86 may get a bit of a free pass here.  AFAIK the QEMU IOMMU
> implementation on x86 has always been "experimental", so it just might
> be okay to change it in a way that causes some older kernels to OOPS.
> 
> --Andy

Since it's experimental, it might be OK to change *guest kernels*
such that they oops on old QEMU.
But guest kernels were not experimental - so we need a QEMU mode that
makes them work fine. The more functionality is available in this QEMU
mode, the betterm because it's going to be the default for a while. For
the same reason, it is preferable to also have new kernels not crash in
this mode.

-- 
MST

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#1382655

On Tue, Apr 19, 2016 at 3:27 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Mon, Apr 18, 2016 at 12:24:15PM -0700, Andy Lutomirski wrote:
>> On Mon, Apr 18, 2016 at 11:29 AM, David Woodhouse <dwmw2@infradead.org> wrote:
>> > For x86, you *can* enable virtio-behind-IOMMU if your DMAR tables tell
>> > the truth, and even legacy kernels ought to cope with that.
>> > FSVO 'ought to' where I suspect some of them will actually crash with a
>> > NULL pointer dereference if there's no "catch-all" DMAR unit in the
>> > tables, which puts it back into the same camp as ARM and Power.
>>
>> I think x86 may get a bit of a free pass here.  AFAIK the QEMU IOMMU
>> implementation on x86 has always been "experimental", so it just might
>> be okay to change it in a way that causes some older kernels to OOPS.
>>
>> --Andy
>
> Since it's experimental, it might be OK to change *guest kernels*
> such that they oops on old QEMU.
> But guest kernels were not experimental - so we need a QEMU mode that
> makes them work fine. The more functionality is available in this QEMU
> mode, the betterm because it's going to be the default for a while. For
> the same reason, it is preferable to also have new kernels not crash in
> this mode.
>

People add QEMU features that need new guest kernels all time time.
If you enable virtio-scsi and try to boot a guest that's too old, it
won't work.  So I don't see anything fundamentally wrong with saying
that the non-experimental QEMU Q35 IOMMU mode won't boot if the guest
kernel is too old.  It might be annoying, since old kernels do work on
actual Q35 hardware, but it at least seems to be that it might be
okay.

--Andy

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#1382662

On Tue, Apr 19, 2016 at 09:02:14AM -0700, Andy Lutomirski wrote:
> On Tue, Apr 19, 2016 at 3:27 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Mon, Apr 18, 2016 at 12:24:15PM -0700, Andy Lutomirski wrote:
> >> On Mon, Apr 18, 2016 at 11:29 AM, David Woodhouse <dwmw2@infradead.org> wrote:
> >> > For x86, you *can* enable virtio-behind-IOMMU if your DMAR tables tell
> >> > the truth, and even legacy kernels ought to cope with that.
> >> > FSVO 'ought to' where I suspect some of them will actually crash with a
> >> > NULL pointer dereference if there's no "catch-all" DMAR unit in the
> >> > tables, which puts it back into the same camp as ARM and Power.
> >>
> >> I think x86 may get a bit of a free pass here.  AFAIK the QEMU IOMMU
> >> implementation on x86 has always been "experimental", so it just might
> >> be okay to change it in a way that causes some older kernels to OOPS.
> >>
> >> --Andy
> >
> > Since it's experimental, it might be OK to change *guest kernels*
> > such that they oops on old QEMU.
> > But guest kernels were not experimental - so we need a QEMU mode that
> > makes them work fine. The more functionality is available in this QEMU
> > mode, the betterm because it's going to be the default for a while. For
> > the same reason, it is preferable to also have new kernels not crash in
> > this mode.
> >
> 
> People add QEMU features that need new guest kernels all time time.
> If you enable virtio-scsi and try to boot a guest that's too old, it
> won't work.  So I don't see anything fundamentally wrong with saying
> that the non-experimental QEMU Q35 IOMMU mode won't boot if the guest
> kernel is too old.  It might be annoying, since old kernels do work on
> actual Q35 hardware, but it at least seems to be that it might be
> okay.
> 
> --Andy

Yes but we need a mode that makes both old and new kernels work, and
that should be the default for a while.  this is what the
IOMMU_PASSTHROUGH flag was about: old kernels ignore it and bypass DMA
API, new kernels go "oh compatibility mode" and bypass the IOMMU
within DMA API.

-- 
MST

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#1382670

On Tue, Apr 19, 2016 at 9:09 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Tue, Apr 19, 2016 at 09:02:14AM -0700, Andy Lutomirski wrote:
>> On Tue, Apr 19, 2016 at 3:27 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> > On Mon, Apr 18, 2016 at 12:24:15PM -0700, Andy Lutomirski wrote:
>> >> On Mon, Apr 18, 2016 at 11:29 AM, David Woodhouse <dwmw2@infradead.org> wrote:
>> >> > For x86, you *can* enable virtio-behind-IOMMU if your DMAR tables tell
>> >> > the truth, and even legacy kernels ought to cope with that.
>> >> > FSVO 'ought to' where I suspect some of them will actually crash with a
>> >> > NULL pointer dereference if there's no "catch-all" DMAR unit in the
>> >> > tables, which puts it back into the same camp as ARM and Power.
>> >>
>> >> I think x86 may get a bit of a free pass here.  AFAIK the QEMU IOMMU
>> >> implementation on x86 has always been "experimental", so it just might
>> >> be okay to change it in a way that causes some older kernels to OOPS.
>> >>
>> >> --Andy
>> >
>> > Since it's experimental, it might be OK to change *guest kernels*
>> > such that they oops on old QEMU.
>> > But guest kernels were not experimental - so we need a QEMU mode that
>> > makes them work fine. The more functionality is available in this QEMU
>> > mode, the betterm because it's going to be the default for a while. For
>> > the same reason, it is preferable to also have new kernels not crash in
>> > this mode.
>> >
>>
>> People add QEMU features that need new guest kernels all time time.
>> If you enable virtio-scsi and try to boot a guest that's too old, it
>> won't work.  So I don't see anything fundamentally wrong with saying
>> that the non-experimental QEMU Q35 IOMMU mode won't boot if the guest
>> kernel is too old.  It might be annoying, since old kernels do work on
>> actual Q35 hardware, but it at least seems to be that it might be
>> okay.
>>
>> --Andy
>
> Yes but we need a mode that makes both old and new kernels work, and
> that should be the default for a while.  this is what the
> IOMMU_PASSTHROUGH flag was about: old kernels ignore it and bypass DMA
> API, new kernels go "oh compatibility mode" and bypass the IOMMU
> within DMA API.

I thought that PLATFORM served that purpose.  Woudn't the host
advertise PLATFORM support and, if the guest doesn't ack it, the host
device would skip translation?  Or is that problematic for vfio?

>
> --
> MST



-- 
Andy Lutomirski
AMA Capital Management, LLC

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#1382682

On Tue, Apr 19, 2016 at 09:12:03AM -0700, Andy Lutomirski wrote:
> On Tue, Apr 19, 2016 at 9:09 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Tue, Apr 19, 2016 at 09:02:14AM -0700, Andy Lutomirski wrote:
> >> On Tue, Apr 19, 2016 at 3:27 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >> > On Mon, Apr 18, 2016 at 12:24:15PM -0700, Andy Lutomirski wrote:
> >> >> On Mon, Apr 18, 2016 at 11:29 AM, David Woodhouse <dwmw2@infradead.org> wrote:
> >> >> > For x86, you *can* enable virtio-behind-IOMMU if your DMAR tables tell
> >> >> > the truth, and even legacy kernels ought to cope with that.
> >> >> > FSVO 'ought to' where I suspect some of them will actually crash with a
> >> >> > NULL pointer dereference if there's no "catch-all" DMAR unit in the
> >> >> > tables, which puts it back into the same camp as ARM and Power.
> >> >>
> >> >> I think x86 may get a bit of a free pass here.  AFAIK the QEMU IOMMU
> >> >> implementation on x86 has always been "experimental", so it just might
> >> >> be okay to change it in a way that causes some older kernels to OOPS.
> >> >>
> >> >> --Andy
> >> >
> >> > Since it's experimental, it might be OK to change *guest kernels*
> >> > such that they oops on old QEMU.
> >> > But guest kernels were not experimental - so we need a QEMU mode that
> >> > makes them work fine. The more functionality is available in this QEMU
> >> > mode, the betterm because it's going to be the default for a while. For
> >> > the same reason, it is preferable to also have new kernels not crash in
> >> > this mode.
> >> >
> >>
> >> People add QEMU features that need new guest kernels all time time.
> >> If you enable virtio-scsi and try to boot a guest that's too old, it
> >> won't work.  So I don't see anything fundamentally wrong with saying
> >> that the non-experimental QEMU Q35 IOMMU mode won't boot if the guest
> >> kernel is too old.  It might be annoying, since old kernels do work on
> >> actual Q35 hardware, but it at least seems to be that it might be
> >> okay.
> >>
> >> --Andy
> >
> > Yes but we need a mode that makes both old and new kernels work, and
> > that should be the default for a while.  this is what the
> > IOMMU_PASSTHROUGH flag was about: old kernels ignore it and bypass DMA
> > API, new kernels go "oh compatibility mode" and bypass the IOMMU
> > within DMA API.
> 
> I thought that PLATFORM served that purpose.  Woudn't the host
> advertise PLATFORM support and, if the guest doesn't ack it, the host
> device would skip translation?  Or is that problematic for vfio?

Exactly that's problematic for security.
You can't allow guest driver to decide whether device skips security.

> >
> > --
> > MST
> 
> 
> 
> -- 
> Andy Lutomirski
> AMA Capital Management, LLC

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#1382756

[Multipart message — attachments visible in raw view] — view raw

On Tue, 2016-04-19 at 19:20 +0300, Michael S. Tsirkin wrote:
> 
> > I thought that PLATFORM served that purpose.  Woudn't the host
> > advertise PLATFORM support and, if the guest doesn't ack it, the host
> > device would skip translation?  Or is that problematic for vfio?
> 
> Exactly that's problematic for security.
> You can't allow guest driver to decide whether device skips security.

Right. Because fundamentally, this *isn't* a property of the endpoint
device, and doesn't live in virtio itself.

It's a property of the platform IOMMU, and lives there.

-- 
dwmw2

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#1382769

On Tue, Apr 19, 2016 at 12:26:44PM -0400, David Woodhouse wrote:
> On Tue, 2016-04-19 at 19:20 +0300, Michael S. Tsirkin wrote:
> > 
> > > I thought that PLATFORM served that purpose.  Woudn't the host
> > > advertise PLATFORM support and, if the guest doesn't ack it, the host
> > > device would skip translation?  Or is that problematic for vfio?
> > 
> > Exactly that's problematic for security.
> > You can't allow guest driver to decide whether device skips security.
> 
> Right. Because fundamentally, this *isn't* a property of the endpoint
> device, and doesn't live in virtio itself.
> 
> It's a property of the platform IOMMU, and lives there.

It's a property of the hypervisor virtio implementation, and lives there.

-- 
MST

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#1382770

On Tue, Apr 19, 2016 at 10:49 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Tue, Apr 19, 2016 at 12:26:44PM -0400, David Woodhouse wrote:
>> On Tue, 2016-04-19 at 19:20 +0300, Michael S. Tsirkin wrote:
>> >
>> > > I thought that PLATFORM served that purpose.  Woudn't the host
>> > > advertise PLATFORM support and, if the guest doesn't ack it, the host
>> > > device would skip translation?  Or is that problematic for vfio?
>> >
>> > Exactly that's problematic for security.
>> > You can't allow guest driver to decide whether device skips security.
>>
>> Right. Because fundamentally, this *isn't* a property of the endpoint
>> device, and doesn't live in virtio itself.
>>
>> It's a property of the platform IOMMU, and lives there.
>
> It's a property of the hypervisor virtio implementation, and lives there.

It is now, but QEMU could, in principle, change the way it thinks
about it so that virtio devices would use the QEMU DMA API but ask
QEMU to pass everything through 1:1.  This would be entirely invisible
to guests but would make it be a property of the IOMMU implementation.
At that point, maybe QEMU could find a (platform dependent) way to
tell the guest what's going on.

FWIW, as far as I can tell, PPC and SPARC really could, in principle,
set up 1:1 mappings in the guest so that the virtio devices would work
regardless of whether QEMU is ignoring the IOMMU or not -- I think the
only obstacle is that the PPC and SPARC 1:1 mappings are currectly set
up with an offset.  I don't know too much about those platforms, but
presumably the layout could be changed so that 1:1 really was 1:1.

--Andy

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#1382823

On Tue, Apr 19, 2016 at 11:01:38AM -0700, Andy Lutomirski wrote:
> On Tue, Apr 19, 2016 at 10:49 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Tue, Apr 19, 2016 at 12:26:44PM -0400, David Woodhouse wrote:
> >> On Tue, 2016-04-19 at 19:20 +0300, Michael S. Tsirkin wrote:
> >> >
> >> > > I thought that PLATFORM served that purpose.  Woudn't the host
> >> > > advertise PLATFORM support and, if the guest doesn't ack it, the host
> >> > > device would skip translation?  Or is that problematic for vfio?
> >> >
> >> > Exactly that's problematic for security.
> >> > You can't allow guest driver to decide whether device skips security.
> >>
> >> Right. Because fundamentally, this *isn't* a property of the endpoint
> >> device, and doesn't live in virtio itself.
> >>
> >> It's a property of the platform IOMMU, and lives there.
> >
> > It's a property of the hypervisor virtio implementation, and lives there.
> 
> It is now, but QEMU could, in principle, change the way it thinks
> about it so that virtio devices would use the QEMU DMA API but ask
> QEMU to pass everything through 1:1.  This would be entirely invisible
> to guests but would make it be a property of the IOMMU implementation.
> At that point, maybe QEMU could find a (platform dependent) way to
> tell the guest what's going on.
> 
> FWIW, as far as I can tell, PPC and SPARC really could, in principle,
> set up 1:1 mappings in the guest so that the virtio devices would work
> regardless of whether QEMU is ignoring the IOMMU or not -- I think the
> only obstacle is that the PPC and SPARC 1:1 mappings are currectly set
> up with an offset.  I don't know too much about those platforms, but
> presumably the layout could be changed so that 1:1 really was 1:1.
> 
> --Andy

Sure. Do you see any reason why the decision to do this can't be
keyed off the virtio feature bit?

-- 
MST

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