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Tsirkin" Cc: Albert Huang , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org Subject: [PATCH v2] virtio_ring: don't update event idx on get_buf Date: Sat, 25 Mar 2023 18:56:33 +0800 Message-Id: <20230325105633.58592-1-huangjie.albert@bytedance.com> X-Mailer: git-send-email 2.37.0 (Apple Git-136) MIME-Version: 1.0 X-Spam-Status: No, score=-0.2 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1761337486535201806?= X-GMAIL-MSGID: =?utf-8?q?1761337486535201806?= 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 will 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, if event_triggered is set to true, do not update vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap 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. Fixes: 8d622d21d248 ("virtio: fix up virtio_disable_cb") Signed-off-by: Albert Huang Message-Id: <20230321085953.24949-1-huangjie.albert@bytedance.com> Signed-off-by: Michael S. Tsirkin Reviewed-by: Xuan Zhuo --- 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 cbeeea1b0439..1c36fa477966 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -914,7 +914,8 @@ 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 (unlikely(!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) && + !vq->event_triggered)) virtio_store_mb(vq->weak_barriers, &vring_used_event(&vq->split.vring), cpu_to_virtio16(_vq->vdev, vq->last_used_idx)); @@ -1744,7 +1745,8 @@ 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 (unlikely(vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC && + !vq->event_triggered)) virtio_store_mb(vq->weak_barriers, &vq->packed.vring.driver->off_wrap, cpu_to_le16(vq->last_used_idx));