Re: [RFC v2 6/7] virtio: in order support for virtio_ring

[Guo Zhi]

----- Original Message -----
> From: "Xuan Zhuo" <xuanzhuo@xxxxxxxxxxxxxxxxx>
> To: "Guo Zhi" <qtxuning1999@xxxxxxxxxxx>
> Cc: "netdev" <netdev@xxxxxxxxxxxxxxx>, "linux-kernel" <linux-kernel@xxxxxxxxxxxxxxx>, "kvm list" <kvm@xxxxxxxxxxxxxxx>,
> "virtualization" <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>, "Guo Zhi" <qtxuning1999@xxxxxxxxxxx>, "eperezma"
> <eperezma@xxxxxxxxxx>, "jasowang" <jasowang@xxxxxxxxxx>, "sgarzare" <sgarzare@xxxxxxxxxx>, "Michael Tsirkin"
> <mst@xxxxxxxxxx>
> Sent: Thursday, August 18, 2022 11:11:58 AM
> Subject: Re: [RFC v2 6/7] virtio: in order support for virtio_ring

> On Wed, 17 Aug 2022 21:57:17 +0800, Guo Zhi <qtxuning1999@xxxxxxxxxxx> wrote:
>> If in order feature negotiated, we can skip the used ring to get
>> buffer's desc id sequentially.
>>
>> Signed-off-by: Guo Zhi <qtxuning1999@xxxxxxxxxxx>
>> ---
>>  drivers/virtio/virtio_ring.c | 53 ++++++++++++++++++++++++++++++------
>>  1 file changed, 45 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> index 1c1b3fa376a2..143184ebb5a1 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_desc_begin;
>>  		} split;
>>
>>  		/* Available for packed ring */
>> @@ -702,8 +705,13 @@ 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;
>> +	/* In order feature use desc in order,
>> +	 * that means, the next desc will always be free
>> +	 */
>> +	if (!virtio_has_feature(vq->vq.vdev, VIRTIO_F_IN_ORDER)) {
> 
> Call virtio_has_feature() here is not good.
> 
> Thanks.
> 

Maybe I can use a variable like vq->indiret?
Thanks.

>> +		vq->split.desc_extra[i].next = vq->free_head;
>> +		vq->free_head = head;
>> +	}
>>
>>  	/* Plus final descriptor */
>>  	vq->vq.num_free++;
>> @@ -745,7 +753,7 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue
>> *_vq,
>>  {
>>  	struct vring_virtqueue *vq = to_vvq(_vq);
>>  	void *ret;
>> -	unsigned int i;
>> +	unsigned int i, j;
>>  	u16 last_used;
>>
>>  	START_USE(vq);
>> @@ -764,11 +772,38 @@ 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_desc_begin;
>> +		j = i;
>> +		/* Indirect only takes one descriptor in descriptor table */
>> +		while (!vq->indirect && (vq->split.desc_extra[j].flags & VRING_DESC_F_NEXT))
>> +			j = (j + 1) % vq->split.vring.num;
>> +		/* move to next */
>> +		j = (j + 1) % vq->split.vring.num;
>> +		/* Next buffer will use this descriptor in order */
>> +		vq->split.next_desc_begin = j;
>> +		if (!vq->indirect) {
>> +			*len = vq->split.desc_extra[i].len;
>> +		} else {
>> +			struct vring_desc *indir_desc =
>> +				vq->split.desc_state[i].indir_desc;
>> +			u32 indir_num = vq->split.desc_extra[i].len, buffer_len = 0;
>> +
>> +			if (indir_desc) {
>> +				for (j = 0; j < indir_num / sizeof(struct vring_desc); j++)
>> +					buffer_len += indir_desc[j].len;
>> +			}
>> +
>> +			*len = buffer_len;
>> +		}
>> +	} 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);
>> @@ -2236,6 +2271,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int
>> index,
>>  	vq->split.avail_flags_shadow = 0;
>>  	vq->split.avail_idx_shadow = 0;
>>
>> +	vq->split.next_desc_begin = 0;
>> +
>>  	/* No callback?  Tell other side not to bother us. */
>>  	if (!callback) {
>>  		vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
>> --
>> 2.17.1
>>