Re: [Qemu-devel] [RFC PATCH] virtio: set ISR status when delivering queue MSI

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 05, 2016 at 05:15:01PM +0200, Ladi Prosek wrote:
> On Thu, May 5, 2016 at 5:02 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Thu, May 05, 2016 at 04:59:13PM +0200, Ladi Prosek wrote:
> >> On Thu, May 5, 2016 at 3:36 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >> > On Thu, May 05, 2016 at 11:13:37AM +0200, Ladi Prosek wrote:
> >> >> There is a discrepancy between dataplane and no-dataplane virtio
> >> >> behavior with respect to the ISR status register and MSI-X
> >> >> capability.
> >> >>
> >> >> Without dataplane the Queue interrupt ISR status bit is set
> >> >> regardless of how the notification is delivered to the guest.
> >> >>
> >> >> With dataplane the Queue interrupt ISR status bit is set only
> >> >> if the notification is delivered as an IRQ.
> >> >>
> >> >> While both conform to the spec, QEMU should be consistent to
> >> >> minimize surprises with broken drivers.
> >> >>
> >> >> This RFC patch shows the relevant code location and contains a
> >> >> possible fix which makes QEMU set the bit on the MSI dataplane
> >> >> code path.
> >> >>
> >> >> Signed-off-by: Ladi Prosek <lprosek@redhat.com>
> >> >
> >> > How is this supposed to work when injecting interrupts using irqfd?
> >> > If anything, I would rather drop setting the flag on non-dataplane.
> >>
> >> My bad, this is messed up. For some reason it works, or appears to be
> >> working at least.
> >>
> >> The reason why I'd like to add setting the flag in dataplane is that
> >> if we go the other way we'll break drivers. I know for sure that there
> >> are drivers depending on this.
> >
> > Rly? These are way out of spec then. I'd rather have these drivers
> > fixed: virtio said ISR can't be used with MSI since the 1st version
> > where MSI was introduced. virtio 1.0 allows using ISR for config
> > interrupts only.
> 
> Should the below paragraph be worded differently then? Or explicitly
> say that the contents of the status word is undefined?

It says don't access. How would you word a driver requirement otherwise?

> Specified since
> 1st version vs. actually behaving since 1st version.

Yea. Yack. People just try whatever seems to work.

> If you think it's
> safe to remove the |= 0x01 from the regular path I'll fix the drivers
> I know of, that's not a problem.

Maybe we should keep the  |= 0x01 in QEMU for now, but let's
not mess up dataplace.  Please fix the drivers.

Maybe remove |= 0x01 if legacy is disabled.

> 4.1.4.5.2 Driver Requirements: ISR status capability
> 
> If MSI-X capability is enabled, the driver SHOULD NOT access ISR
> status upon detecting a Queue Interrupt.
> 
> >> >
> >> >> ---
> >> >>  hw/virtio/virtio.c | 19 ++++++++++++++-----
> >> >>  1 file changed, 14 insertions(+), 5 deletions(-)
> >> >>
> >> >> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> >> >> index 08275a9..4053313 100644
> >> >> --- a/hw/virtio/virtio.c
> >> >> +++ b/hw/virtio/virtio.c
> >> >> @@ -1192,7 +1192,7 @@ void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
> >> >>      }
> >> >>
> >> >>      trace_virtio_notify(vdev, vq);
> >> >> -    vdev->isr |= 0x01;
> >> >> +    vdev->isr |= 0x01; // <= here we set ISR status even if we don't raise IRQ
> >> >>      virtio_notify_vector(vdev, vq->vector);
> >> >>  }
> >> >>
> >> >> @@ -1757,16 +1757,25 @@ static void virtio_queue_guest_notifier_read(EventNotifier *n)
> >> >>      }
> >> >>  }
> >> >>
> >> >> +static void virtio_queue_guest_notifier_read_irqfd(EventNotifier *n)
> >> >> +{
> >> >> +    VirtQueue *vq = container_of(n, VirtQueue, guest_notifier);
> >> >> +    if (event_notifier_test_and_clear(n)) {
> >> >> +        vq->vdev->isr |= 0x01;
> >> >> +    }
> >> >> +}
> >> >> +
> >> >>  void virtio_queue_set_guest_notifier_fd_handler(VirtQueue *vq, bool assign,
> >> >>                                                  bool with_irqfd)
> >> >>  {
> >> >> -    if (assign && !with_irqfd) {
> >> >> +    if (assign) {
> >> >>          event_notifier_set_handler(&vq->guest_notifier,
> >> >> -                                   virtio_queue_guest_notifier_read);
> >> >> +             with_irqfd ?
> >> >> +                virtio_queue_guest_notifier_read_irqfd :
> >> >> +                virtio_queue_guest_notifier_read);
> >> >>      } else {
> >> >>          event_notifier_set_handler(&vq->guest_notifier, NULL);
> >> >> -    }
> >> >> -    if (!assign) {
> >> >> +
> >> >>          /* Test and clear notifier before closing it,
> >> >>           * in case poll callback didn't have time to run. */
> >> >>          virtio_queue_guest_notifier_read(&vq->guest_notifier);
> >> >> --
> >> >> 2.5.5