[PATCH RFC v2 1/4] virtio: fix up virtio_disable_cb

Michael S. Tsirkin
2021-04-13T05:48:00Z

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>
---
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