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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 30 Mar 2023 10:04:03 -0400

On Thu, Mar 30, 2023 at 02:54:21PM +0800, Xuan Zhuo wrote:
> On Thu, 30 Mar 2023 02:44:44 -0400, "Michael S. Tsirkin" <[email-masked]> wrote:
> > 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.
> 
> I agree with these codes:
> 
> -		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
> 
> 
> I just don't understand event_triggered.

The comment near it says it all:
        /* Hint for event idx: already triggered no need to disable. */
the write into event idx is potentially expensive since it can
invalidate cache for another processor (depending on the CPU).

> >
> > 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.
> 
> Sorry, I just have some problems, I don't oppose it. At least it can reduce the
> modification of vring_used_event(&vq->split.vring). I think it is also beneficial.
> 
> Thanks very much.
> 
> 
> >
> > > >
> > > > 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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> >