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Tsirkin" To: Sasha Levin Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, virtualization@lists.linux-foundation.org Subject: Re: [PATCH AUTOSEL 5.4 28/28] virtio: fix up virtio_disable_cb Message-ID: <20210711002423-mutt-send-email-mst@kernel.org> References: <20210710235107.3221840-1-sashal@kernel.org> <20210710235107.3221840-28-sashal@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210710235107.3221840-28-sashal@kernel.org> Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Sat, Jul 10, 2021 at 07:51:07PM -0400, Sasha Levin wrote: > From: "Michael S. Tsirkin" > > [ Upstream commit 8d622d21d24803408b256d96463eac4574dcf067 ] > > 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. > > Signed-off-by: Michael S. Tsirkin > Signed-off-by: Sasha Levin I don't think we should have it in stable just yet. Let's make sure it fixes the issue first. > --- > 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 97e8a195e18f..b6e1e0598529 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 > -- > 2.30.2