linux-kernel - [PATCH v3] virtio_ring: interrupt disable flag updated to vq even with event

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Message-Id: <20230329072135.44757-1-huangjie.albert@bytedance.com>
Date:   Wed, 29 Mar 2023 15:21:35 +0800
From:   Albert Huang <huangjie.albert@...edance.com>
To:     Jason Wang <jasowang@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>
Cc:     "huangjie.albert" <huangjie.albert@...edance.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3] virtio_ring: interrupt disable flag updated to vq even with event_triggered is set

From: "huangjie.albert" <huangjie.albert@...edance.com>

in virtio_net, if we disable the napi_tx, when we triger a tx interrupt,
the vq->event_triggered will be set to true. It will no longer be set to
false. Unless we explicitly call virtqueue_enable_cb_delayed or
virtqueue_enable_cb_prepare.

If we disable the napi_tx, it will only be called when the tx ring
buffer is relatively small.

Because event_triggered is true. Therefore, VRING_AVAIL_F_NO_INTERRUPT or
VRING_PACKED_EVENT_FLAG_DISABLE will not be set. So we update
vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap
every time we call virtqueue_get_buf_ctx.This bring more interruptions.

To summarize:
1) event_triggered was set to true in vring_interrupt()
2) after this nothing will happen for virtqueue_disable_cb() so
   VRING_AVAIL_F_NO_INTERRUPT is not set in avail_flags_shadow
3) virtqueue_get_buf_ctx_split() will still think the cb is enabled
   then it tries to publish new event

To fix:
update VRING_AVAIL_F_NO_INTERRUPT or VRING_PACKED_EVENT_FLAG_DISABLE to vq
when we call virtqueue_disable_cb even the event_triggered is set to true.

Tested with iperf:
iperf3 tcp stream:
vm1 -----------------> vm2
vm2 just receives tcp data stream from vm1, and sends the ack to vm1,
there are many tx interrupts in vm2.
but without event_triggered there are just a few tx interrupts.

v2->v3:
-update the interrupt disable flag even with the event_triggered is set,
-instead of checking whether event_triggered is set in
-virtqueue_get_buf_ctx_{packed/split}, will cause the drivers  which have
-not called virtqueue_{enable/disable}_cb to miss notifications.

Fixes: 8d622d21d248 ("virtio: fix up virtio_disable_cb")
Signed-off-by: huangjie.albert <huangjie.albert@...edance.com>
Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
Signed-off-by: Jason Wang <jasowang@...hat.com>
---
 drivers/virtio/virtio_ring.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 307e139cb11d..ad74463a48ee 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -812,6 +812,14 @@ 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 device triggered an event already it won't trigger one again:
+		 * no need to disable.
+		 */
+		if (vq->event_triggered)
+			return;
+
 		if (vq->event)
 			/* TODO: this is a hack. Figure out a cleaner value to write. */
 			vring_used_event(&vq->split.vring) = 0x0;
@@ -1544,8 +1552,16 @@ static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
-	if (vq->packed.event_flags_shadow != VRING_PACKED_EVENT_FLAG_DISABLE) {
+	if (!(vq->packed.event_flags_shadow & VRING_PACKED_EVENT_FLAG_DISABLE)) {
 		vq->packed.event_flags_shadow = VRING_PACKED_EVENT_FLAG_DISABLE;
+
+		/*
+		 * If device triggered an event already it won't trigger one again:
+		 * no need to disable.
+		 */
+		if (vq->event_triggered)
+			return;
+
 		vq->packed.vring.driver->flags =
 			cpu_to_le16(vq->packed.event_flags_shadow);
 	}
@@ -2063,12 +2079,6 @@ 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
-- 
2.31.1