Re: [PATCH 0/8][v2] MSI-X mask emulation support for assigned device

[Avi Kivity <avi@redhat.com>]

  On 10/20/2010 05:22 PM, Alex Williamson wrote:
> On Wed, 2010-10-20 at 17:17 +0200, Avi Kivity wrote:
> >  On 10/20/2010 03:43 PM, Michael S. Tsirkin wrote:
> >  >  >   >If instead of eventfd we had a file descriptor that can pass vector
> >  >  >   >information from vfio to kvm and back, that would fix it,
> >  >  >   >as we would not need to set us GSIs at all,
> >  >  >   >and not need for userspace to handle MSIX specially.
> >  >  >   >
> >  >  >
> >  >  >   But if we emulate the entire msix bar in vfio, that's not needed, right?
> >  >
> >  >  Yes, I think it is. How does kvm know which interrupt to inject?
> >  >  Either vfio needs to pass that info to qemu and qemu would pass it
> >  >  to kvm, or vfio would have some way to pass that info to kvm
> >  >  directly.
> >
> >  Wait.  We can't emulate the BAR in vfio, we have to emulate it in kvm
> >  where we emulate the write instruction.  We then need to tell vfio,
> >  perhaps via userspace, that masking state has changed.
> >
> >  Seems very intrusive.
>
> We wouldn't direct map the vector table or pending bits, so we could
> trap and emulate in qemu, which could then call into reads/writes in
> vfio.
>

That's 100% unintrusive for kvm, but that's what we do today, which is 
deemed too slow.

Another option is to fake an interrupt remapping device and do a direct 
map.  Will those older guests recognize and use it?  I imagine not.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.