Re: [PATCH] virtio_blk: always post notifications under the lock

[Christian Borntraeger <borntraeger@linux.ibm.com>]

Am 07.01.25 um 19:25 schrieb Andrew Boyer:
> Commit af8ececda185 ("virtio: add VIRTIO_F_NOTIFICATION_DATA feature
> support") added notification data support to the core virtio driver
> code. When this feature is enabled, the notification includes the
> updated producer index for the queue. Thus it is now critical that
> notifications arrive in order.
> 
> The virtio_blk driver has historically not worried about notification
> ordering. Modify it so that the prepare and kick steps are both done
> under the vq lock.
> 
> Signed-off-by: Andrew Boyer <andrew.boyer@amd.com>
> Reviewed-by: Brett Creeley <brett.creeley@amd.com>
> Fixes: af8ececda185 ("virtio: add VIRTIO_F_NOTIFICATION_DATA feature support")
> Cc: Viktor Prutyanov <viktor@daynix.com>
> Cc: virtualization@lists.linux.dev
> Cc: linux-block@vger.kernel.org
> ---
>   drivers/block/virtio_blk.c | 19 ++++---------------
>   1 file changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 3efe378f1386..14d9e66bb844 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -379,14 +379,10 @@ static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
>   {
>   	struct virtio_blk *vblk = hctx->queue->queuedata;
>   	struct virtio_blk_vq *vq = &vblk->vqs[hctx->queue_num];
> -	bool kick;
>   
>   	spin_lock_irq(&vq->lock);
> -	kick = virtqueue_kick_prepare(vq->vq);
> +	virtqueue_kick(vq->vq);
>   	spin_unlock_irq(&vq->lock);
> -
> -	if (kick)
> -		virtqueue_notify(vq->vq);
>   }

I would assume this will be a performance nightmare for normal IO.