Re: [Qemu-devel] [PATCH] vfio-pci: Fix Nvidia MSI ACK through 0x88000 quirk

[Dave Airlie <airlied@gmail.com>]

On Tue, Nov 12, 2013 at 8:32 AM, Alex Williamson
<alex.williamson@redhat.com> wrote:
> On Tue, 2013-11-12 at 07:55 +1000, Dave Airlie wrote:
>> On Tue, Nov 12, 2013 at 7:43 AM, Alex Williamson
>> <alex.williamson@redhat.com> wrote:
>> > When MSI is enabled on Nvidia GeForce cards the driver seems to
>> > acknowledge the interrupt by writing a 0xff byte to the MSI capability
>> > ID register using the PCI config space mirror at offset 0x88000 from
>> > BAR0.  Without this, the device will only fire a single interrupt.
>> > VFIO handles the PCI capability ID/next registers as virtual w/o write
>> > support, so any write through config space is currently dropped.  Add
>> > a check for this and allow the write through the BAR window.  The
>> > registers are read-only anyway.
>>
>> This is only half the truth, I'm afraid if I'm right its much worse than that.
>>
>> At least on some GPUs the MSI ack is done via PCI config space itself,
>> and on some its done via the mirror, and yes it matters on some cards
>> which way it works.
>
> I was hoping that wouldn't be the case since it seems fairly universal
> that PCI config space access should be considered slow and avoided for
> things like this.  But, I suppose with MMConfig it's no worse than
> device MMIO space.

For reference dig around in here

http://www.mail-archive.com/nouveau@lists.freedesktop.org/msg14437.html

nvidia commented on it as well, it may be they require the slowness or
just some other coherency issue.

Dave.