Re: KVM devices assignment; PCIe AER?

Michael S. Tsirkin
2010-10-28T04:58:18Z

On Wed, Oct 27, 2010 at 04:58:20PM -0600, Alex Williamson wrote:
> On Wed, 2010-10-27 at 14:43 -0700, Etienne Martineau wrote:
> > On Wed, 27 Oct 2010, Alex Williamson wrote:
> > > No, emulated devices trigger interrupts directly with qemu_set_irq.
> > > irqfds are currently only used by vhost afaik, since it's being
> > > interrupted externally, much like pass through devices are.
> >
> > Fair enough. Thanks for the clarification.
> >
> > > Sort of. When the VFIO device triggers an interrupt, we get notified
> > > via the eventfd we've registered for that interrupt. We can then call
> > > qemu_set_irq directly to raise that interrupt in the KVM kernel APIC.
> > > That much works today.
> >
> > Understood but performance wise this is no good for KVM right?
>
> Right, bouncing interrupts and EOIs through qemu via eventfds is going
> to add latency. On the interrupt path we already have irqfds, which
> will avoid the bounce through userspace, we just need to use them.
> Doing something similar with EOIs could avoid that path, giving us
> something comparable to current device assignment.
>
> > > The irqfd mechanism is simply a way for KVM to
> > > directly consume the eventfd and raise an interrupt via a pre-setup
> > > vector. That's yet to be implemented for INTx on VFIO, but should
> > > mostly be a matter of connecting existing pieces together. It's working
> > > for MSI-X.
> >
> > OK, I was on the impression you already had irqfd 'connected' to KVM from
> > VFIO... This is why I was asking about the nature of the changed in VFIO.
> >
> > > When VFIO sends an interrupt, it disables the physical device from
> > > generating more interrupts (this is where VFIO requires PCI 2.3
> > > compliant devices for the INTx disable bit int he status register).
> > > When the guest services the interrupt, we can detect this by catching
> > > the EOI of the IOAPIC. At that point, we can re-eanble interrupts on
> > > the device. Wash, rinse, repeat.
> > >
> > > To do this in qemu, I created a callback on the ioapic where drivers can
> > > register for the interrupt they care about. Since KVM moves the ioapic
> > > into the kernel, we need to extend this into KVM and have yet another
> > > eventfd mechanism. It's possible that we could have the VFIO kernel
> > > module also receive this eventfd, re-enabling interrupts on the device,
> > > in much the same way as above.
> >
> > In the cases of KVM where are you going to catch the EIO? For some
> > reason I'm on the impression that this is part of KVM. If so then how are
> > you going to 'signal' to VFIO? Cannot use eventfd here right?
>
> KVM already has an internal IRQ ACK notifier (which is what current
> device assignment uses to do the same thing), it's just a matter of
> adding a callback that does a kvm_register_irq_ack_notifier that sends
> off the eventfd signal. I've got this working and will probably send
> out the KVM patch this week. For now the eventfd goes to userspace, but
> this is where I imagine we could steal some of the irqfd code to make
> VFIO consume the irqfd signal directly. Thanks,
>
> Alex

BTW, how do we handle sharing the interrupt in guest?

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