From:   Albert Huang <huangjie.albert@bytedance.com>
To:     "Michael S . Tsirkin " <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>
Cc:     "huangjie.albert" <huangjie.albert@bytedance.com>,
        virtualization@lists.linux-foundation.org (open list:VIRTIO CORE AND
        NET DRIVERS), linux-kernel@vger.kernel.org (open list)
Subject: [PATCH] virtio_ring: Suppress tx interrupt when napi_tx disable
Date:   Tue, 21 Mar 2023 16:59:53 +0800
Message-Id: <20230321085953.24949-1-huangjie.albert@bytedance.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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

fix commit 8d622d21d248 ("virtio: fix up virtio_disable_cb")

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:
virtio_net->start_xmit:
	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
		netif_stop_subqueue(dev, qnum);
		if (!use_napi &&
		    unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
			/* More just got used, free them then recheck. */
			free_old_xmit_skbs(sq, false);
			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
				netif_start_subqueue(dev, qnum);
				virtqueue_disable_cb(sq->vq);
			}
		}
	}
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 will bring more interruptions.

if event_triggered is set to true, do not update vring_used_event(&vq->split.vring)
or vq->packed.vring.driver->off_wrap

Signed-off-by: huangjie.albert <huangjie.albert@bytedance.com>
---
 drivers/virtio/virtio_ring.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 307e139cb11d..f486cccadbeb 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -795,7 +795,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
 	/* If we expect an interrupt for the next entry, tell host
 	 * by writing event index and flush out the write before
 	 * the read in the next get_buf call. */
-	if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
+	if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)
+			&& (vq->event_triggered == false))
 		virtio_store_mb(vq->weak_barriers,
 				&vring_used_event(&vq->split.vring),
 				cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
@@ -1529,7 +1530,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
 	 * by writing event index and flush out the write before
 	 * the read in the next get_buf call.
 	 */
-	if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC)
+	if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC
+			&& (vq->event_triggered == false))
 		virtio_store_mb(vq->weak_barriers,
 				&vq->packed.vring.driver->off_wrap,
 				cpu_to_le16(vq->last_used_idx));
-- 
2.31.1