linux-kernel - Re: [PATCH 2/2] virtio_ring: add unlikely annotation for free descs check

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Message-ID: <20220329075024.eoajm5ufrcfytug4@sgarzare-redhat>
Date:   Tue, 29 Mar 2022 09:50:24 +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 2/2] virtio_ring: add unlikely annotation for free descs
 check

On Mon, Mar 28, 2022 at 06:58:17PM +0800, Xianting Tian wrote:
>The 'if (vq->vq.num_free < descs_used)' check will almost always be false.
>
>Signed-off-by: Xianting Tian <xianting.tian@...ux.alibaba.com>
>---
> drivers/virtio/virtio_ring.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>

>
>diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>index d597fc0874ec..ab6d5f0cb579 100644
>--- a/drivers/virtio/virtio_ring.c
>+++ b/drivers/virtio/virtio_ring.c
>@@ -525,7 +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
>-- 
>2.17.1
>