Subject: Re: [PATCH v3 3/4] virtio: fix up virtio_disable_cb

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 30 Mar 2023 02:44:44 -0400

On Thu, Mar 30, 2023 at 02:07:37PM +0800, Xuan Zhuo wrote:
> On Wed, 26 May 2021 04:24:40 -0400, "Michael S. Tsirkin" <[email-masked]> wrote:
> > virtio_disable_cb is currently a nop for split ring with event index.
> > This is because it used to be always called from a callback when we know
> > device won't trigger more events until we update the index.  However,
> > now that we run with interrupts enabled a lot we also poll without a
> > callback so that is different: disabling callbacks will help reduce the
> > number of spurious interrupts.
> > Further, if using event index with a packed ring, and if being called
> > from a callback, we actually do disable interrupts which is unnecessary.
> >
> > Fix both issues by tracking whenever we get a callback. If that is
> > the case disabling interrupts with event index can be a nop.
> > If not the case disable interrupts. Note: with a split ring
> > there's no explicit "no interrupts" value. For now we write
> > a fixed value so our chance of triggering an interupt
> > is 1/ring size. It's probably better to write something
> > related to the last used index there to reduce the chance
> > even further. For now I'm keeping it simple.
> 
> 
> Don't understand, is this patch necessary? For this patch set, we can do without
> this patch.
> 
> So doest this patch optimize virtqueue_disable_cb() by reducing a modification of
> vring_used_event(&vq-> split.vring)?
> 
> Or I miss something.
> 
> Thanks.

Before this patch virtqueue_disable_cb did nothing at all
for the common case of event index enabled, so
calling it from virtio net would not help matters.

But the patch is from 2021, isn't it a bit too late to argue?
If you have a cleanup or an optimization in mind, please
post a patch.

> >
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> >  drivers/virtio/virtio_ring.c | 26 +++++++++++++++++++++++++-
> >  1 file changed, 25 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 71e16b53e9c1..88f0b16b11b8 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -113,6 +113,9 @@ struct vring_virtqueue {
> >  	/* Last used index we've seen. */
> >  	u16 last_used_idx;
> >
> > +	/* Hint for event idx: already triggered no need to disable. */
> > +	bool event_triggered;
> > +
> >  	union {
> >  		/* Available for split ring */
> >  		struct {
> > @@ -739,7 +742,10 @@ 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 (!vq->event)
> > +		if (vq->event)
> > +			/* TODO: this is a hack. Figure out a cleaner value to write. */
> > +			vring_used_event(&vq->split.vring) = 0x0;
> > +		else
> >  			vq->split.vring.avail->flags =
> >  				cpu_to_virtio16(_vq->vdev,
> >  						vq->split.avail_flags_shadow);
> > @@ -1605,6 +1611,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
> >  	vq->weak_barriers = weak_barriers;
> >  	vq->broken = false;
> >  	vq->last_used_idx = 0;
> > +	vq->event_triggered = false;
> >  	vq->num_added = 0;
> >  	vq->packed_ring = true;
> >  	vq->use_dma_api = vring_use_dma_api(vdev);
> > @@ -1919,6 +1926,12 @@ 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
> > @@ -1942,6 +1955,9 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> >  {
> >  	struct vring_virtqueue *vq = to_vvq(_vq);
> >
> > +	if (vq->event_triggered)
> > +		vq->event_triggered = false;
> > +
> >  	return vq->packed_ring ? virtqueue_enable_cb_prepare_packed(_vq) :
> >  				 virtqueue_enable_cb_prepare_split(_vq);
> >  }
> > @@ -2005,6 +2021,9 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
> >  {
> >  	struct vring_virtqueue *vq = to_vvq(_vq);
> >
> > +	if (vq->event_triggered)
> > +		vq->event_triggered = false;
> > +
> >  	return vq->packed_ring ? virtqueue_enable_cb_delayed_packed(_vq) :
> >  				 virtqueue_enable_cb_delayed_split(_vq);
> >  }
> > @@ -2044,6 +2063,10 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
> >  	if (unlikely(vq->broken))
> >  		return IRQ_HANDLED;
> >
> > +	/* Just a hint for performance: so it's ok that this can be racy! */
> > +	if (vq->event)
> > +		vq->event_triggered = true;
> > +
> >  	pr_debug("virtqueue callback for %p (%p)\n", vq, vq->vq.callback);
> >  	if (vq->vq.callback)
> >  		vq->vq.callback(&vq->vq);
> > @@ -2083,6 +2106,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> >  	vq->weak_barriers = weak_barriers;
> >  	vq->broken = false;
> >  	vq->last_used_idx = 0;
> > +	vq->event_triggered = false;
> >  	vq->num_added = 0;
> >  	vq->use_dma_api = vring_use_dma_api(vdev);
> >  #ifdef DEBUG
> > --
> > MST
> >
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