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

Alex Williamson
2010-10-20T15:38:34Z

On Wed, 2010-10-20 at 17:26 +0200, Avi Kivity wrote:
> 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.

Nope, I think it's a non-starter to think anything but the latest
current guests will have support for that.

Alex