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

Avi Kivity
2010-10-20T15:54:26Z

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

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