Re: [PATCH AUTOSEL 5.12 42/43] virtio: fix up virtio_disable_cb

On Sat, Jul 10, 2021 at 07:49:14PM -0400, Sasha Levin wrote:

From: "Michael S. Tsirkin" mst@redhat.com

[ 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 mst@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org

I am not sure we want this in stable yet. It should in theory fix the reported interrupt storms but the reporter is on vacation.

---

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

2.30.2