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

[Alex Williamson <alex.williamson@redhat.com>]

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.

Alex

> We can perhaps use the new semaphore capability in eventfd to pass 
> mask/unmask information from kvm to userspace.  We'd usually hook it to 
> the corresponding vfio facility.
> 
> > >  How far away is vfio?  If it's merged soon, we might avoid making
> > >  changes to the old assigned device infrastructure and instead update
> > >  vfio.
> >
> > Hard to be sure, hopefully 2.6.38 material.
> 
> My question was how to implement this with vfio.  But your list is 
> interesting as well.
> 
>