Re: [PATCH] virtio_ring: aovid reading flag from the descriptor ring

["Michael S. Tsirkin" <mst@redhat.com>]

Typo in the subject btw.

minor tweaks to commit log below

On Mon, Nov 08, 2021 at 04:13:24PM +0800, Jason Wang wrote:
> Commit 72b5e8958738 ("virtio-ring: store DMA metadata in desc_extra
> for split virtqueue") tries to make it possible for the driver to not
> read from the descriptor ring to prevent the device from corrupting
> the descriptor ring. But it still read 

reads

>the descriptor flag from the
> descriptor ring during buffer detach.
> 
> This patch fixes 

fixes this

>by always store 

storing

>the descriptor flag no matter whether
> DMA API is used and then we can avoid reading descriptor flag from the
> descriptor ring. This eliminates the possibly of unexpected next
> descriptor caused by the wrong flag (e.g the next flag).
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>


I'd also like the commit log to document what the issue is in a bit more depth.
I think the main reason we are checking the dma API is this


static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
                                          unsigned int i)
{
        struct vring_desc_extra *extra = vq->split.desc_extra;
        u16 flags;

        if (!vq->use_dma_api)
                goto out;

	...
}


so I guess with a bad flag, what will happen is num_free will become too
big is that right?




> ---
>  drivers/virtio/virtio_ring.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 00f64f2f8b72..28734f4e57d3 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -583,7 +583,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>  	}
>  	/* Last one doesn't continue. */
>  	desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> -	if (!indirect && vq->use_dma_api)
> +	if (!indirect)
>  		vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
>  			~VRING_DESC_F_NEXT;
>  

BTW I'm a bit confused why we need the & (vq->split.vring.num - 1) logic.
Maybe it's time we stopped writing out descriptor then overwriting it -
e.g. return the desc_extra pointer from virtqueue_add_desc_split
instead of an index. Worth checking what this does to performance.


> @@ -713,7 +713,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
>  	/* Put back on free list: unmap first-level descriptors and find end */
>  	i = head;
>  
> -	while (vq->split.vring.desc[i].flags & nextflag) {
> +	while (vq->split.desc_extra[i].flags & nextflag) {
>  		vring_unmap_one_split(vq, i);
>  		i = vq->split.desc_extra[i].next;
>  		vq->vq.num_free++;
> -- 
> 2.25.1

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