linux-kernel - Re: [PATCH] virtio_ring: remove unnecessary to

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Date:   Mon, 28 Mar 2022 11:05:03 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Xianting Tian <xianting.tian@...ux.alibaba.com>
Cc:     mst@...hat.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio_ring: remove unnecessary to_vvq call in vring hot
 path

On Thu, Mar 24, 2022 at 03:33:40PM +0800, Xianting Tian wrote:
>It passes '_vq' to virtqueue_use_indirect(), which still calls
>to_vvq to get 'vq', let's directly pass 'vq'. It can avoid
>unnecessary call of to_vvq in hot path.

It seems reasonable to me.

>
>Other tiny optimization:
>Add unlikely to "if (vq->vq.num_free < descs_used).

Better to do this change in another patch.

Thanks,
Stefano

>
>Signed-off-by: Xianting Tian <xianting.tian@...ux.alibaba.com>
>---
> drivers/virtio/virtio_ring.c | 10 ++++------
> 1 file changed, 4 insertions(+), 6 deletions(-)
>
>diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>index 962f1477b1fa..ab6d5f0cb579 100644
>--- a/drivers/virtio/virtio_ring.c
>+++ b/drivers/virtio/virtio_ring.c
>@@ -205,11 +205,9 @@ struct vring_virtqueue {
>
> #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
>
>-static inline bool virtqueue_use_indirect(struct virtqueue *_vq,
>+static inline bool virtqueue_use_indirect(struct vring_virtqueue *vq,
> 					  unsigned int total_sg)
> {
>-	struct vring_virtqueue *vq = to_vvq(_vq);
>-
> 	/*
> 	 * If the host supports indirect descriptor tables, and we have multiple
> 	 * buffers, then go indirect. FIXME: tune this threshold
>@@ -507,7 +505,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>
> 	head = vq->free_head;
>
>-	if (virtqueue_use_indirect(_vq, total_sg))
>+	if (virtqueue_use_indirect(vq, total_sg))
> 		desc = alloc_indirect_split(_vq, total_sg, gfp);
> 	else {
> 		desc = NULL;
>@@ -527,7 +525,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> 		descs_used = total_sg;
> 	}
>
>-	if (vq->vq.num_free < descs_used) {
>+	if (unlikely(vq->vq.num_free < descs_used)) {
> 		pr_debug("Can't add buf len %i - avail = %i\n",
> 			 descs_used, vq->vq.num_free);
> 		/* FIXME: for historical reasons, we force a notify here if
>@@ -1194,7 +1192,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>
> 	BUG_ON(total_sg == 0);
>
>-	if (virtqueue_use_indirect(_vq, total_sg)) {
>+	if (virtqueue_use_indirect(vq, total_sg)) {
> 		err = virtqueue_add_indirect_packed(vq, sgs, total_sg, out_sgs,
> 						    in_sgs, data, gfp);
> 		if (err != -ENOMEM) {
>-- 
>2.17.1
>