linux-kernel - [RFC PATCH 01/17] virtio_ring: Avoid reading unneeded used length

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Message-Id: <20210517090836.533-2-xieyongji@bytedance.com>
Date:   Mon, 17 May 2021 17:08:20 +0800
From:   Xie Yongji <xieyongji@...edance.com>
To:     mst@...hat.com, jasowang@...hat.com, stefanha@...hat.com
Cc:     amit@...nel.org, arei.gonglei@...wei.com, airlied@...ux.ie,
        kraxel@...hat.com, dan.j.williams@...el.com,
        johannes@...solutions.net, ohad@...ery.com,
        bjorn.andersson@...aro.org, david@...hat.com, vgoyal@...hat.com,
        miklos@...redi.hu, sgarzare@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: [RFC PATCH 01/17] virtio_ring: Avoid reading unneeded used length

If device driver doesn't need used length, it can pass a NULL
len in virtqueue_get_buf()/virtqueue_get_buf_ctx(). Then
we can avoid reading and validating the len value in used
ring entries.

Signed-off-by: Xie Yongji <xieyongji@...edance.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 54e12dd91310..d999a1d6d271 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -772,7 +772,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
 	last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
 	i = virtio32_to_cpu(_vq->vdev,
 			vq->split.vring.used->ring[last_used].id);
-	*len = virtio32_to_cpu(_vq->vdev,
+	if (len)
+		*len = virtio32_to_cpu(_vq->vdev,
 			vq->split.vring.used->ring[last_used].len);
 
 	if (unlikely(i >= vq->split.vring.num)) {
@@ -1444,7 +1445,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
 
 	last_used = vq->last_used_idx;
 	id = le16_to_cpu(vq->packed.vring.desc[last_used].id);
-	*len = le32_to_cpu(vq->packed.vring.desc[last_used].len);
+	if (len)
+		*len = le32_to_cpu(vq->packed.vring.desc[last_used].len);
 
 	if (unlikely(id >= vq->packed.vring.num)) {
 		BAD_RING(vq, "id %u out of range\n", id);
-- 
2.11.0