Linux Virtualization - Mar 2023 - [External] Re: [PATCH v3] virtio_ring: interrupt disable flag updated to vq even with event_triggered is set

On Wed, 29 Mar 2023 17:33:23 +0800, =?utf-8?b?6buE5p2w?= <huangjie.albert at
bytedance.com> wrote:
> Xuan Zhuo <xuanzhuo at linux.alibaba.com> ?2023?3?29??? 17:27???
> >
> > On Wed, 29 Mar 2023 15:28:41 +0800, Albert Huang <huangjie.albert
at bytedance.com> wrote:
> > > From: "huangjie.albert" <huangjie.albert at
bytedance.com>
> > >
> > > in virtio_net, if we disable the napi_tx, when we triger a tx
interrupt,
> > > the vq->event_triggered will be set to true. It will no longer
be set to
> > > false. Unless we explicitly call virtqueue_enable_cb_delayed or
> > > virtqueue_enable_cb_prepare.
> > >
> > > If we disable the napi_tx, it will only be called when the tx
ring
> > > buffer is relatively small.
> > >
> > > Because event_triggered is true. Therefore,
VRING_AVAIL_F_NO_INTERRUPT or
> > > VRING_PACKED_EVENT_FLAG_DISABLE will not be set. So we update
> > > vring_used_event(&vq->split.vring) or
vq->packed.vring.driver->off_wrap
> > > every time we call virtqueue_get_buf_ctx.This bring more
interruptions.
> > >
> > > To summarize:
> > > 1) event_triggered was set to true in vring_interrupt()
> > > 2) after this nothing will happen for virtqueue_disable_cb() so
> > >    VRING_AVAIL_F_NO_INTERRUPT is not set in avail_flags_shadow
> > > 3) virtqueue_get_buf_ctx_split() will still think the cb is
enabled
> > >    then it tries to publish new event
> > >
> > > To fix:
> > > update VRING_AVAIL_F_NO_INTERRUPT or
VRING_PACKED_EVENT_FLAG_DISABLE to vq
> > > when we call virtqueue_disable_cb even the event_triggered is set
to true.
> > >
> > > Tested with iperf:
> > > iperf3 tcp stream:
> > > vm1 -----------------> vm2
> > > vm2 just receives tcp data stream from vm1, and sends the ack to
vm1,
> > > there are many tx interrupts in vm2.
> > > but without event_triggered there are just a few tx interrupts.
> > >
> > > v2->v3:
> > > -update the interrupt disable flag even with the event_triggered
is set,
> > > -instead of checking whether event_triggered is set in
> > > -virtqueue_get_buf_ctx_{packed/split}, will cause the drivers 
which have
> > > -not called virtqueue_{enable/disable}_cb to miss notifications.
> > >
> > > Fixes: 8d622d21d248 ("virtio: fix up
virtio_disable_cb")
> > > Signed-off-by: huangjie.albert <huangjie.albert at
bytedance.com>
> > > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> > > Signed-off-by: Jason Wang <jasowang at redhat.com>
> > > ---
> > >  drivers/virtio/virtio_ring.c | 24 +++++++++++++++++-------
> > >  1 file changed, 17 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c
b/drivers/virtio/virtio_ring.c
> > > index 307e139cb11d..ad74463a48ee 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -812,6 +812,14 @@ static void
virtqueue_disable_cb_split(struct virtqueue *_vq)
> > >
> > >       if (!(vq->split.avail_flags_shadow &
VRING_AVAIL_F_NO_INTERRUPT)) {
> > >               vq->split.avail_flags_shadow |=
VRING_AVAIL_F_NO_INTERRUPT;
> > > +
> > > +             /*
> > > +              * If device triggered an event already it
won't trigger one again:
> > > +              * no need to disable.
> > > +              */
> > > +             if (vq->event_triggered)
> > > +                     return;
> > > +
> > >               if (vq->event)
> > >                       /* TODO: this is a hack. Figure out a
cleaner value to write. */
> > >                       vring_used_event(&vq->split.vring) =
0x0;
> > > @@ -1544,8 +1552,16 @@ static void
virtqueue_disable_cb_packed(struct virtqueue *_vq)
> > >  {
> > >       struct vring_virtqueue *vq = to_vvq(_vq);
> > >
> > > -     if (vq->packed.event_flags_shadow !=
VRING_PACKED_EVENT_FLAG_DISABLE) {
> > > +     if (!(vq->packed.event_flags_shadow &
VRING_PACKED_EVENT_FLAG_DISABLE)) {
> >
> > This seems to be another problem.
> >
> > Thanks.
> >
>
> Here, we are only making this change to maintain consistency with
> virtqueue_disable_cb_split.
> Is there any concern with doing so?
I'm not sure why the use_flags_shadow use "!=" to judge, but it
seems
that all places are used like this. So we'd better keep the original. If it
is
necessary to modify it here, I think a separate commit should be posted.
Even if it is just to keep it consistent with split.


Thanks.


>
> Thanks.
>
> >
> > >               vq->packed.event_flags_shadow =
VRING_PACKED_EVENT_FLAG_DISABLE;
> > > +
> > > +             /*
> > > +              * If device triggered an event already it
won't trigger one again:
> > > +              * no need to disable.
> > > +              */
> > > +             if (vq->event_triggered)
> > > +                     return;
> > > +
> > >               vq->packed.vring.driver->flags > > >
cpu_to_le16(vq->packed.event_flags_shadow);
> > >       }
> > > @@ -2063,12 +2079,6 @@ void virtqueue_disable_cb(struct virtqueue
*_vq)
> > >  {
> > >       struct vring_virtqueue *vq = to_vvq(_vq);
> > >
> > > -     /* If device triggered an event already it won't
trigger one again:
> > > -      * no need to disable.
> > > -      */
> > > -     if (vq->event_triggered)
> > > -             return;
> > > -
> > >       if (vq->packed_ring)
> > >               virtqueue_disable_cb_packed(_vq);
> > >       else
> > > --
> > > 2.31.1
> > >