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Groups > linux.kernel > #1257707 > unrolled thread

[PATCH v3 0/3] virtio DMA API core stuff

Started byAndy Lutomirski <luto@kernel.org>
First post2015-10-28 07:40 +0100
Last post2015-10-28 15:10 +0100
Articles 6 on this page of 26 — 7 participants

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  [PATCH v3 0/3] virtio DMA API core stuff Andy Lutomirski <luto@kernel.org> - 2015-10-28 07:40 +0100
    [PATCH v3 2/3] virtio_ring: Support DMA APIs Andy Lutomirski <luto@kernel.org> - 2015-10-28 07:50 +0100
    [PATCH v3 3/3] virtio_pci: Use the DMA API Andy Lutomirski <luto@kernel.org> - 2015-10-28 07:50 +0100
    [PATCH v3 1/3] virtio_net: Stop doing DMA from the stack Andy Lutomirski <luto@kernel.org> - 2015-10-28 07:50 +0100
      Re: [PATCH v3 1/3] virtio_net: Stop doing DMA from the stack "Michael S. Tsirkin" <mst@redhat.com> - 2015-10-28 08:10 +0100
    Re: [PATCH v3 0/3] virtio DMA API core stuff David Woodhouse <dwmw2@infradead.org> - 2015-10-28 08:00 +0100
      Re: [PATCH v3 0/3] virtio DMA API core stuff Andy Lutomirski <luto@amacapital.net> - 2015-10-28 08:20 +0100
    Re: [PATCH v3 0/3] virtio DMA API core stuff "Michael S. Tsirkin" <mst@redhat.com> - 2015-10-28 08:20 +0100
      Re: [PATCH v3 0/3] virtio DMA API core stuff Christian Borntraeger <borntraeger@de.ibm.com> - 2015-10-28 08:50 +0100
        Re: [PATCH v3 0/3] virtio DMA API core stuff David Woodhouse <dwmw2@infradead.org> - 2015-10-28 09:20 +0100
          Re: [PATCH v3 0/3] virtio DMA API core stuff "Michael S. Tsirkin" <mst@redhat.com> - 2015-10-28 12:40 +0100
            Re: [PATCH v3 0/3] virtio DMA API core stuff David Woodhouse <dwmw2@infradead.org> - 2015-10-28 14:40 +0100
              Re: [PATCH v3 0/3] virtio DMA API core stuff "Michael S. Tsirkin" <mst@redhat.com> - 2015-10-28 15:10 +0100
                Re: [PATCH v3 0/3] virtio DMA API core stuff David Woodhouse <dwmw2@infradead.org> - 2015-10-28 15:20 +0100
                  Re: [PATCH v3 0/3] virtio DMA API core stuff "Michael S. Tsirkin" <mst@redhat.com> - 2015-10-28 15:30 +0100
                    Re: [PATCH v3 0/3] virtio DMA API core stuff David Woodhouse <dwmw2@infradead.org> - 2015-10-28 15:40 +0100
                      Re: [PATCH v3 0/3] virtio DMA API core stuff "Michael S. Tsirkin" <mst@redhat.com> - 2015-10-28 17:20 +0100
                        Re: [PATCH v3 0/3] virtio DMA API core stuff Andy Lutomirski <luto@amacapital.net> - 2015-10-29 00:00 +0100
                          Re: [PATCH v3 0/3] virtio DMA API core stuff "Michael S. Tsirkin" <mst@redhat.com> - 2015-10-29 10:10 +0100
                            Re: [PATCH v3 0/3] virtio DMA API core stuff David Woodhouse <dwmw2@infradead.org> - 2015-10-29 17:20 +0100
                            Re: [PATCH v3 0/3] virtio DMA API core stuff Joerg Roedel <jroedel@suse.de> - 2015-10-30 16:20 +0100
                            Re: [PATCH v3 0/3] virtio DMA API core stuff David Woodhouse <dwmw2@infradead.org> - 2015-10-30 18:00 +0100
        Re: [PATCH v3 0/3] virtio DMA API core stuff Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-10-28 09:40 +0100
          Re: [PATCH v3 0/3] virtio DMA API core stuff "Michael S. Tsirkin" <mst@redhat.com> - 2015-10-28 12:30 +0100
            Re: [PATCH v3 0/3] virtio DMA API core stuff David Woodhouse <dwmw2@infradead.org> - 2015-10-28 14:40 +0100
              Re: [PATCH v3 0/3] virtio DMA API core stuff "Michael S. Tsirkin" <mst@redhat.com> - 2015-10-28 15:10 +0100

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#1259569

FromJoerg Roedel <jroedel@suse.de>
Date2015-10-30 16:20 +0100
Message-ID<qpjDc-7r6-17@gated-at.bofh.it>
In reply to#1258668
On Thu, Oct 29, 2015 at 11:01:41AM +0200, Michael S. Tsirkin wrote:
> Example: you have a mix of assigned devices and virtio devices. You
> don't trust your assigned device vendor not to corrupt your memory so
> you want to limit the damage your assigned device can do to your guest,
> so you use an IOMMU for that.  Thus existing iommu=pt within guest is out.
> 
> But you trust your hypervisor (you have no choice anyway),
> and you don't want the overhead of tweaking IOMMU
> on data path for virtio. Thus iommu=on is out too.

IOMMUs on x86 usually come with an ACPI table that describes which
IOMMUs are in the system and which devices they translate. So you can
easily describe all devices there that are not behind an IOMMU.

The ACPI table is built by the BIOS, and the platform intialization code
sets the device dma_ops accordingly. If the BIOS provides wrong
information in the ACPI table this is a platform bug.

> I'm not sure what ACPI has to do with it.  It's about a way for guest
> users to specify whether they want to bypass an IOMMU for a given
> device.

We have no way yet to request passthrough-mode per-device from the IOMMU
drivers, but that can easily be added. But as I see it:

> By the way, a bunch of code is missing on the QEMU side
> to make this useful:
> 1. virtio ignores the iommu
> 2. vhost user ignores the iommu
> 3. dataplane ignores the iommu
> 4. vhost-net ignores the iommu
> 5. VFIO ignores the iommu

Qemu does not implement IOMMU translation for virtio devices anyway
(which is fine), so it just should tell the guest so in the ACPI table
built to describe the emulated IOMMU.


	Joerg

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#1259629

FromDavid Woodhouse <dwmw2@infradead.org>
Date2015-10-30 18:00 +0100
Message-ID<qplbY-8eF-23@gated-at.bofh.it>
In reply to#1258668

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

(Sorry, missed part of this before).

On Thu, 2015-10-29 at 11:01 +0200, Michael S. Tsirkin wrote:
> Isn't this specified by the hypervisor? I don't think this is a good
> way to do this: guest security should be up to guest.

And it is. When the guest sees an IOMMU, it can choose to use it, or
choose not to (or choose to put it in passthrough mode). But as Jörg
says, we don't have a way for an individual  device driver to *request*
passthrough mode or not yet; the choice is made by the core IOMMU code
(iommu=pt on the command line) — or by the platform simply stating that
a given device isn't *covered* by an IOMMU, if that is indeed the case.

In *no* circumstance is it sane for a device driver just to "opt out"
of using the correct DMA API function calls, and expect that to
*magically* cause the IOMMU to be bypassed.

> > Everyone seems to agree that x86's emulated Q35 thing
> > is just buggy right now and should be taught to use the existing ACPI
> > mechanism for enumerating passthrough devices.
> 
> I'm not sure what ACPI has to do with it.
> It's about a way for guest users to specify whether
> they want to bypass an IOMMU for a given device.

No, it absolutely isn't. You might want that — and see the discussion
about DMA_ATTR_IOMMU_BYPASS if you do. But that is *utterly* irrelevant
to *this* discussion, in which you seem to be advocating that the
virtio drivers should remain buggy by just unilaterally not using the
DMA API.

> By the way, a bunch of code is missing on the QEMU side
> to make this useful:
> 1. virtio ignores the iommu
> 2. vhost user ignores the iommu
> 3. dataplane ignores the iommu
> 4. vhost-net ignores the iommu
> 5. VFIO ignores the iommu

No, those things are not useful for fixing the virtio driver bug under
discussion here. All we need to do is make the virtio drivers correctly
use the DMA API. They should never have passed review and been accepted
into the Linux kernel without that.

All we need to do first is make sure that the bug we have in the
PowerPC IOMMU code (and potentially ARM and/or SPARC?) is fixed, and
that it doesn't attempt to use an IOMMU that doesn't exist. And ensure
that the virtualised IOMMU on qemu/x86 isn't lying and claiming that it
translates for the virtio devices when it doesn't.

There are other things we might want to do — like fixing the IOMMU that
qemu can emulate, and actually making it work with real assigned
devices (currently it's totally hosed because it doesn't handle that
case at all). And potentially making the virtualised IOMMU actually
*do* translation for virtio devices (as opposed to just admitting
correctly that it doesn't). But those aren't strictly relevant here,
yet.

It's not clear what specific uses of the IOMMU you had in mind in your
above list — could you elucidate?

-- 
dwmw2

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#1257790

FromBenjamin Herrenschmidt <benh@kernel.crashing.org>
Date2015-10-28 09:40 +0100
Message-ID<qouqZ-dw-5@gated-at.bofh.it>
In reply to#1257746
On Wed, 2015-10-28 at 16:40 +0900, Christian Borntraeger wrote:
> We have discussed that at kernel summit. I will try to implement a dummy dma_ops for
> s390 that does 1:1 mapping and Ben will look into doing some quirk to handle "old"
> code in addition to also make it possible to mark devices as iommu bypass (IIRC,
> via device tree, Ben?)

Something like that yes. I'll look into it when I'm back home.

Cheers,
Ben.

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#1257927

From"Michael S. Tsirkin" <mst@redhat.com>
Date2015-10-28 12:30 +0100
Message-ID<qox5x-1Y5-37@gated-at.bofh.it>
In reply to#1257790
On Wed, Oct 28, 2015 at 05:36:53PM +0900, Benjamin Herrenschmidt wrote:
> On Wed, 2015-10-28 at 16:40 +0900, Christian Borntraeger wrote:
> > We have discussed that at kernel summit. I will try to implement a dummy dma_ops for
> > s390 that does 1:1 mapping and Ben will look into doing some quirk to handle "old"
> > code in addition to also make it possible to mark devices as iommu bypass (IIRC,
> > via device tree, Ben?)
> 
> Something like that yes. I'll look into it when I'm back home.
> 
> Cheers,
> Ben.

OK so I guess that means we should prefer a transport-specific
interface in virtio-pci then.

-- 
MST
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#1258029

FromDavid Woodhouse <dwmw2@infradead.org>
Date2015-10-28 14:40 +0100
Message-ID<qoz7l-3cS-39@gated-at.bofh.it>
In reply to#1257927

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

On Wed, 2015-10-28 at 13:23 +0200, Michael S. Tsirkin wrote:
> On Wed, Oct 28, 2015 at 05:36:53PM +0900, Benjamin Herrenschmidt
> wrote:
> > On Wed, 2015-10-28 at 16:40 +0900, Christian Borntraeger wrote:
> > > We have discussed that at kernel summit. I will try to implement
> > > a dummy dma_ops for
> > > s390 that does 1:1 mapping and Ben will look into doing some
> > > quirk to handle "old"
> > > code in addition to also make it possible to mark devices as
> > > iommu bypass (IIRC,
> > > via device tree, Ben?)
> > 
> > Something like that yes. I'll look into it when I'm back home.
> > 
> > Cheers,
> > Ben.
> 
> OK so I guess that means we should prefer a transport-specific
> interface in virtio-pci then.

Why?

-- 
dwmw2

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#1258079

From"Michael S. Tsirkin" <mst@redhat.com>
Date2015-10-28 15:10 +0100
Message-ID<qozAl-3CV-1@gated-at.bofh.it>
In reply to#1258029
On Wed, Oct 28, 2015 at 10:37:56PM +0900, David Woodhouse wrote:
> On Wed, 2015-10-28 at 13:23 +0200, Michael S. Tsirkin wrote:
> > On Wed, Oct 28, 2015 at 05:36:53PM +0900, Benjamin Herrenschmidt
> > wrote:
> > > On Wed, 2015-10-28 at 16:40 +0900, Christian Borntraeger wrote:
> > > > We have discussed that at kernel summit. I will try to implement
> > > > a dummy dma_ops for
> > > > s390 that does 1:1 mapping and Ben will look into doing some
> > > > quirk to handle "old"
> > > > code in addition to also make it possible to mark devices as
> > > > iommu bypass (IIRC,
> > > > via device tree, Ben?)
> > > 
> > > Something like that yes. I'll look into it when I'm back home.
> > > 
> > > Cheers,
> > > Ben.
> > 
> > OK so I guess that means we should prefer a transport-specific
> > interface in virtio-pci then.
> 
> Why?

Because you said you are doing something device tree specific for ARM,
aren't you?

-- 
MST
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