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 12:44 +0200, Michael S. Tsirkin wrote:
> On Wed, Oct 20, 2010 at 11:51:01AM +0200, Avi Kivity wrote:
> >  On 10/20/2010 10:26 AM, Sheng Yang wrote:
> > >Here is v2.
> > >
> > >Changelog:
> > >
> > >v1->v2
> > >
> > >The major change from v1 is I've added the in-kernel MSI-X mask emulation
> > >support, as well as adding shortcuts for reading MSI-X table.
> > >
> > >I've taken Michael's advice to use mask/unmask directly, but unsure about
> > >exporting irq_to_desc() for module...
> > >
> > >Also add flush_work() according to Marcelo's comments.
> > >
> > 
> > Any performance numbers?  What are the affected guests?  just RHEL
> > 4, or any others?
> 
> Likely any old linux.
> 
> > Alex, Michael, how would you do this with vfio?
> 
> With current VFIO we would catch mask writes in qemu and
> call a KVM ioctl. We would also need an ioctl to retrieve
> pending bits long term.

Ugh, no.  VFIO us currently independent of KVM.  I'd like to keep it
that way.  We'll need to optimize interrupt injection and eoi via KVM,
but it should only be a performance optimization, not a functional
requirement.

It would probably make sense to request a mask/unmask ioctl in VFIO for
MSI-X, then perhaps the pending bits would only support read/write (no
mmap), so we could avoid an ioctl there.

> I think that it is unfortunate that we need to do this in userspace
> while rest of configuration is done in kernel.
> I would be much happier with userspace simply forwarding
> everything to VFIO, so emulation does not have to
> be split. That would be a clean interface: just mmap
> MSIX BAR and forget about it.
> 
> 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.

Sounds good except that I'd like to get away from forcing device
assignment interfaces into KVM.

Alex