netdev - Re: [RFC 4/5] virtio: get desc id in order

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Date:   Tue, 26 Jul 2022 16:07:46 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Guo Zhi <qtxuning1999@...u.edu.cn>, eperezma@...hat.com,
        sgarzare@...hat.com, mst@...hat.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [RFC 4/5] virtio: get desc id in order


在 2022/7/21 16:43, Guo Zhi 写道:
> If in order feature negotiated, we can skip the used ring to get
> buffer's desc id sequentially.


Let's rename the patch to something like "in order support for virtio_ring"


>
> Signed-off-by: Guo Zhi <qtxuning1999@...u.edu.cn>
> ---
>   drivers/virtio/virtio_ring.c | 37 ++++++++++++++++++++++++++++--------
>   1 file changed, 29 insertions(+), 8 deletions(-)


I don't see packed support in this patch, we need to implement that.


>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index a5ec724c0..4d57a4edc 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -144,6 +144,9 @@ struct vring_virtqueue {
>   			/* DMA address and size information */
>   			dma_addr_t queue_dma_addr;
>   			size_t queue_size_in_bytes;
> +
> +			/* In order feature batch begin here */
> +			u16 next_batch_desc_begin;
>   		} split;
>   
>   		/* Available for packed ring */
> @@ -700,8 +703,10 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
>   	}
>   
>   	vring_unmap_one_split(vq, i);
> -	vq->split.desc_extra[i].next = vq->free_head;
> -	vq->free_head = head;
> +	if (!virtio_has_feature(vq->vq.vdev, VIRTIO_F_IN_ORDER)) {
> +		vq->split.desc_extra[i].next = vq->free_head;
> +		vq->free_head = head;
> +	}


Let's add a comment to explain why we don't need anything if in order is 
neogitated.


>   
>   	/* Plus final descriptor */
>   	vq->vq.num_free++;
> @@ -743,7 +748,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   	void *ret;
> -	unsigned int i;
> +	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
> +	unsigned int i, j;
>   	u16 last_used;
>   
>   	START_USE(vq);
> @@ -762,11 +768,24 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
>   	/* Only get used array entries after they have been exposed by host. */
>   	virtio_rmb(vq->weak_barriers);
>   
> -	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,
> -			vq->split.vring.used->ring[last_used].len);
> +	if (virtio_has_feature(_vq->vdev, VIRTIO_F_IN_ORDER)) {
> +		/* Skip used ring and get used desc in order*/
> +		i = vq->split.next_batch_desc_begin;
> +		j = i;
> +		while (vq->split.vring.desc[j].flags & nextflag)


Let's don't depend on the descriptor ring which is under the control of 
the malicious hypervisor.

Let's use desc_extra that is not visible by the hypervisor. More can be 
seen in this commit:

72b5e8958738 ("virtio-ring: store DMA metadata in desc_extra for split 
virtqueue")


> +			j = (j + 1) % vq->split.vring.num;
> +		/* move to next */
> +		j = (j + 1) % vq->split.vring.num;
> +		vq->split.next_batch_desc_begin = j;


I'm not sure I get the logic here, basically I think we should check 
buffer instead of descriptor here.

So if vring.used->ring[last_used].id != last_used, we know all 
[last_used, vring.used->ring[last_used].id] have been used in a batch?


> +
> +		/* TODO: len of buffer */


So spec said:

"

The skipped buffers (for which no used ring entry was written) are 
assumed to have been used (read or written) by the device completely.


"

Thanks


> +	} else {
> +		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,
> +				       vq->split.vring.used->ring[last_used].len);
> +	}
>   
>   	if (unlikely(i >= vq->split.vring.num)) {
>   		BAD_RING(vq, "id %u out of range\n", i);
> @@ -2234,6 +2253,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
>   	vq->split.avail_flags_shadow = 0;
>   	vq->split.avail_idx_shadow = 0;
>   
> +	vq->split.next_batch_desc_begin = 0;
> +
>   	/* No callback?  Tell other side not to bother us. */
>   	if (!callback) {
>   		vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;