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[RFC PATCH v3 3/5] PCI: Add host bridge attribute to indicate filtering of MSIs is supported

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  [RFC PATCH v3 3/5] PCI: Add host bridge attribute to indicate filtering of MSIs is supported Yongji Xie <xyjxie@linux.vnet.ibm.com> - 2016-01-15 08:10 +0100
    RE: [RFC PATCH v3 3/5] PCI: Add host bridge attribute to indicate  filtering of MSIs is supported David Laight <David.Laight@ACULAB.COM> - 2016-01-15 18:30 +0100
      Re: [RFC PATCH v3 3/5] PCI: Add host bridge attribute to indicate  filtering of MSIs is supported Yongji Xie <xyjxie@linux.vnet.ibm.com> - 2016-01-20 10:50 +0100

#1309913 — [RFC PATCH v3 3/5] PCI: Add host bridge attribute to indicate filtering of MSIs is supported

MSI-X tables are not allowed to be mmapped in vfio-pci
driver in case that user get to touch this directly.
This will cause some performance issues when when PCI
adapters have critical registers in the same page as
the MSI-X table.

However, some kind of PCI host bridge such as IODA bridge
on Power support filtering of MSIs, which can ensure that a
given pci device can only shoot the MSIs assigned for it.
So we think it's safe to expose the MSI-X table to userspace
if filtering of MSIs is supported because the exposed MSI-X
table can't be used to do harm to other memory space.

To support this case, this patch adds a pci_host_bridge
attribute to indicate if this PCI host bridge supports
filtering of MSIs.

Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>
---
 drivers/pci/host-bridge.c |    6 ++++++
 include/linux/pci.h       |    3 +++
 2 files changed, 9 insertions(+)

diff --git a/drivers/pci/host-bridge.c b/drivers/pci/host-bridge.c
index 5f4a2e0..c029267 100644
--- a/drivers/pci/host-bridge.c
+++ b/drivers/pci/host-bridge.c
@@ -96,3 +96,9 @@ void pcibios_bus_to_resource(struct pci_bus *bus, struct resource *res,
 	res->end = region->end + offset;
 }
 EXPORT_SYMBOL(pcibios_bus_to_resource);
+
+bool pci_host_bridge_msi_filtered_enabled(struct pci_dev *pdev)
+{
+	return pci_find_host_bridge(pdev->bus)->msi_filtered;
+}
+EXPORT_SYMBOL_GPL(pci_host_bridge_msi_filtered_enabled);
diff --git a/include/linux/pci.h b/include/linux/pci.h
index b640d65..b952b78 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -412,6 +412,7 @@ struct pci_host_bridge {
 	void (*release_fn)(struct pci_host_bridge *);
 	void *release_data;
 	unsigned int ignore_reset_delay:1;	/* for entire hierarchy */
+	unsigned int msi_filtered:1;	/* support filtering of MSIs */
 	/* Resource alignment requirements */
 	resource_size_t (*align_resource)(struct pci_dev *dev,
 			const struct resource *res,
@@ -430,6 +431,8 @@ void pci_set_host_bridge_release(struct pci_host_bridge *bridge,
 
 int pcibios_root_bridge_prepare(struct pci_host_bridge *bridge);
 
+bool pci_host_bridge_msi_filtered_enabled(struct pci_dev *pdev);
+
 /*
  * The first PCI_BRIDGE_RESOURCE_NUM PCI bus resources (those that correspond
  * to P2P or CardBus bridge windows) go in a table.  Additional ones (for
-- 
1.7.9.5

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#1310300 — RE: [RFC PATCH v3 3/5] PCI: Add host bridge attribute to indicate filtering of MSIs is supported

From: Yongji Xie
> Sent: 15 January 2016 07:06
>
> MSI-X tables are not allowed to be mmapped in vfio-pci
> driver in case that user get to touch this directly.
> This will cause some performance issues when when PCI
> adapters have critical registers in the same page as
> the MSI-X table.
...
If the driver wants to generate an incorrect MSI-X interrupt
it can do so by requesting the device do a normal memory transfer
to the target address area that raises MSI-X interrupts.
So disabling writes to the MSI-X table (and pending bit array)
areas only raises the bar very slightly.
A device may also give the driver write access to the MSI-X
table through other addresses.

This seems to make disallowing the mapping of the MSI-X table
rather pointless.

I've also dumped out the MSI-X table (during development) to
check that the values are being written there correctly.

	David

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#1313040 — Re: [RFC PATCH v3 3/5] PCI: Add host bridge attribute to indicate filtering of MSIs is supported

On 2016/1/16 1:24, David Laight wrote:
> From: Yongji Xie
>> Sent: 15 January 2016 07:06
>>
>> MSI-X tables are not allowed to be mmapped in vfio-pci
>> driver in case that user get to touch this directly.
>> This will cause some performance issues when when PCI
>> adapters have critical registers in the same page as
>> the MSI-X table.
> ...
> If the driver wants to generate an incorrect MSI-X interrupt
> it can do so by requesting the device do a normal memory transfer
> to the target address area that raises MSI-X interrupts.

IOMMUs supporting interrupt remapping can prevent this case.

> So disabling writes to the MSI-X table (and pending bit array)
> areas only raises the bar very slightly.
> A device may also give the driver write access to the MSI-X
> table through other addresses.
>
> This seems to make disallowing the mapping of the MSI-X table
> rather pointless.

If we allow the mapping of the MSI-X table, it seems the guest
kernels of some architectures can write invalid data to MSI-X table
when device drivers initialize MSI-X interrupts.

Regards,
Yongji Xie

> I've also dumped out the MSI-X table (during development) to
> check that the values are being written there correctly.
>
> 	David
>

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