[Qemu-devel] [Bug 1771042] [NEW] dataplane interrupt handler doesn't support msi

[Qemu-devel] [Bug 1771042] [NEW] dataplane interrupt handler doesn't support msi

[eric hoffman]

Public bug reported:

hw/block/dataplane/virtio-blk.c commit
1010cadf62332017648abee0d7a3dc7f2eef9632

in the function notify_guest_bh, the function virtio_notify_irqfd is called
to deliver the interrupt corresponding to the vq

however, without the dataplane, hw/block/virtio_blk_req_complete calls
virtio_notify to deliver the interrupt (immediately). this goes though
a slightly more involved path that calls virtio_pci_notify which includes
a case to handle msi interrupts.

so, msi interrupts with block devices aren't serviced when using dataplane
batching.

** Affects: qemu
     Importance: Undecided
         Status: New

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Title:
  dataplane interrupt handler doesn't support msi

Status in QEMU:
  New

Bug description:
  hw/block/dataplane/virtio-blk.c commit
  1010cadf62332017648abee0d7a3dc7f2eef9632

  in the function notify_guest_bh, the function virtio_notify_irqfd is called
  to deliver the interrupt corresponding to the vq

  however, without the dataplane, hw/block/virtio_blk_req_complete calls
  virtio_notify to deliver the interrupt (immediately). this goes though
  a slightly more involved path that calls virtio_pci_notify which includes
  a case to handle msi interrupts.

  so, msi interrupts with block devices aren't serviced when using dataplane
  batching.

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[Qemu-devel] [Bug 1771042] Re: dataplane interrupt handler doesn't support msi

[eric hoffman]

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 101f32c..31d9eb8 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -73,7 +73,7 @@ static void notify_guest_bh(void *opaque)
             unsigned i = j + ctzl(bits);
             VirtQueue *vq = virtio_get_queue(s->vdev, i);

-            virtio_notify_irqfd(s->vdev, vq);
+            virtio_notify(s->vdev, vq);

             bits &= bits - 1; /* clear right-most bit */
         }

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Title:
  dataplane interrupt handler doesn't support msi

Status in QEMU:
  New

Bug description:
  hw/block/dataplane/virtio-blk.c commit
  1010cadf62332017648abee0d7a3dc7f2eef9632

  in the function notify_guest_bh, the function virtio_notify_irqfd is called
  to deliver the interrupt corresponding to the vq

  however, without the dataplane, hw/block/virtio_blk_req_complete calls
  virtio_notify to deliver the interrupt (immediately). this goes though
  a slightly more involved path that calls virtio_pci_notify which includes
  a case to handle msi interrupts.

  so, msi interrupts with block devices aren't serviced when using dataplane
  batching.

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Re: [Qemu-devel] [Bug 1771042] Re: dataplane interrupt handler doesn't support msi

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 997 bytes --]

On Mon, May 14, 2018 at 03:00:44AM -0000, eric hoffman wrote:
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 101f32c..31d9eb8 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -73,7 +73,7 @@ static void notify_guest_bh(void *opaque)
>              unsigned i = j + ctzl(bits);
>              VirtQueue *vq = virtio_get_queue(s->vdev, i);
> 
> -            virtio_notify_irqfd(s->vdev, vq);
> +            virtio_notify(s->vdev, vq);
> 
>              bits &= bits - 1; /* clear right-most bit */
>          }

Please send patches to qemu-devel@nongnu.org.  Guidelines for submitting
patches are here:
https://wiki.qemu.org/Contribute/SubmitAPatch

The issue with this approach is that hw/pci/msi.c:msi_send_message()
invokes device emulation outside the QEMU global mutex (it calls into
the APIC to send MSIs).  I've CCed Paolo Bonzini to check whether doing
this is thread-safe.

Stefan

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[Qemu-devel] [Bug 1771042] Re: dataplane interrupt handler doesn't support msi

[eric hoffman]

oh right, another note. this only manifests when using kvm.

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Title:
  dataplane interrupt handler doesn't support msi

Status in QEMU:
  New

Bug description:
  hw/block/dataplane/virtio-blk.c commit
  1010cadf62332017648abee0d7a3dc7f2eef9632

  in the function notify_guest_bh, the function virtio_notify_irqfd is called
  to deliver the interrupt corresponding to the vq

  however, without the dataplane, hw/block/virtio_blk_req_complete calls
  virtio_notify to deliver the interrupt (immediately). this goes though
  a slightly more involved path that calls virtio_pci_notify which includes
  a case to handle msi interrupts.

  so, msi interrupts with block devices aren't serviced when using dataplane
  batching.

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[Qemu-devel] [Bug 1771042] Re: dataplane interrupt handler doesn't support msi

[eric hoffman]

thanks for looking at this Stefan - since I don't have any context of
exactly the kind of environmental issues like threading, the patch
posted here isn't really a suggested fix.

it does in general seem helpful if batched interrupts have the same
delivery semantics as non-deferred.

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Title:
  dataplane interrupt handler doesn't support msi

Status in QEMU:
  New

Bug description:
  hw/block/dataplane/virtio-blk.c commit
  1010cadf62332017648abee0d7a3dc7f2eef9632

  in the function notify_guest_bh, the function virtio_notify_irqfd is called
  to deliver the interrupt corresponding to the vq

  however, without the dataplane, hw/block/virtio_blk_req_complete calls
  virtio_notify to deliver the interrupt (immediately). this goes though
  a slightly more involved path that calls virtio_pci_notify which includes
  a case to handle msi interrupts.

  so, msi interrupts with block devices aren't serviced when using dataplane
  batching.

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[Bug 1771042] Re: dataplane interrupt handler doesn't support msi

[Thomas Huth]

** Changed in: qemu
       Status: New => Invalid

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Title:
  dataplane interrupt handler doesn't support msi

Status in QEMU:
  Invalid

Bug description:
  hw/block/dataplane/virtio-blk.c commit
  1010cadf62332017648abee0d7a3dc7f2eef9632

  in the function notify_guest_bh, the function virtio_notify_irqfd is called
  to deliver the interrupt corresponding to the vq

  however, without the dataplane, hw/block/virtio_blk_req_complete calls
  virtio_notify to deliver the interrupt (immediately). this goes though
  a slightly more involved path that calls virtio_pci_notify which includes
  a case to handle msi interrupts.

  so, msi interrupts with block devices aren't serviced when using dataplane
  batching.

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