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

From: Yongji Xie
Date: Wed Jan 20 2016 - 04:42:40 EST

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.

Yongji Xie

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