On Thu, 30 Mar 2023 10:04:03 -0400, "Michael S. Tsirkin" <[email protected]> wrote: > 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 protected]> > > 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 protected]> 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).
Yes, I agree. Thanks. > > > > > > > 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 protected]> > > > > > --- > > > > > 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 > > > > > > > > > > _______________________________________________ > > > > > Virtualization mailing list > > > > > [email protected] > > > > > https://lists.linuxfoundation.org/mailman/listinfo/virtualization > > > > _______________________________________________ Virtualization mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/virtualization
