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

Michael S. Tsirkin
2010-10-20T16:06:16Z

On Wed, Oct 20, 2010 at 05:54:20PM +0200, Avi Kivity wrote:
> On 10/20/2010 05:38 PM, Alex Williamson wrote:
> >> > We'll need to optimize interrupt injection and eoi via KVM,
> >> > but it should only be a performance optimization, not a functional
> >> > requirement.
> >>
> >> For level-triggered interrupts only, yes? MSI EOI does not involve any
> >> device or interrupt controller visible action?
> >
> >Right, the EOI is for legacy interrupts only. Perhaps we don't care
> >enough about performance of those to route it through KVM so long as we
> >can co-exist with the KVM APIC.
>
> We will need a way to get the EOI out to userspace, then.
>
> Perhaps that will give us motivation to split the ioapic from the
> kernel (though I suppose fear of regressions will stop us).
>
> Anyway EOI notifiers are also useful for timekeeping.
>
> > MSIs are currently still bouncing from
> >VFIO to QEMU to the guest, which seems inefficient.
>
> Why is that? Can't you use irqfd like vhost-net?
>
> >> > 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 would much like to see in-band information (which mask/unmask is for
> >> older Linux) done via eventfds so userspace is not involved.
> >
> >Hmm, I'm not sure how to do that yet.
>
> One ugly way is to use two eventfds, one counting mask events, one
> counting unmask events. The difference is the value if the masked
> bit.
>
> Another option is to use
> http://permalink.gmane.org/gmane.linux.kernel.commits.head/188038
> which allows kvm to maintain a counter using eventfd. The vfio
> state machine then looks like:
>
> interrupt:
> if counter == 0 mask interrupts, set pending bit, wait for counter
> to become 1
> if counter == 1 forward it
> counter becomes 1: unmask, forward interrupt if pending

One issue is that this page has a ton of other info.
KVM would have to keep all that in kernel...

--
MST