Re: [RFC v2 1/7] vhost: expose used buffers

[Guo Zhi <qtxuning1999@sjtu.edu.cn>]

----- Original Message -----
> From: "jasowang" <jasowang@redhat.com>
> To: "Guo Zhi" <qtxuning1999@sjtu.edu.cn>, "eperezma" <eperezma@redhat.com>, "sgarzare" <sgarzare@redhat.com>, "Michael
> Tsirkin" <mst@redhat.com>
> Cc: "netdev" <netdev@vger.kernel.org>, "linux-kernel" <linux-kernel@vger.kernel.org>, "kvm list" <kvm@vger.kernel.org>,
> "virtualization" <virtualization@lists.linux-foundation.org>
> Sent: Thursday, August 25, 2022 3:01:31 PM
> Subject: Re: [RFC v2 1/7] vhost: expose used buffers

> 在 2022/8/17 21:57, Guo Zhi 写道:
>> Follow VIRTIO 1.1 spec, only writing out a single used ring for a batch
>> of descriptors.
>>
>> Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
>> ---
>>   drivers/vhost/vhost.c | 14 ++++++++++++--
>>   drivers/vhost/vhost.h |  1 +
>>   2 files changed, 13 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 40097826cff0..7b20fa5a46c3 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2376,10 +2376,20 @@ static int __vhost_add_used_n(struct vhost_virtqueue
>> *vq,
>>   	vring_used_elem_t __user *used;
>>   	u16 old, new;
>>   	int start;
>> +	int copy_n = count;
>>   
>> +	/**
>> +	 * If in order feature negotiated, devices can notify the use of a batch of
>> buffers to
>> +	 * the driver by only writing out a single used ring entry with the id
>> corresponding
>> +	 * to the head entry of the descriptor chain describing the last buffer in the
>> batch.
>> +	 */
>> +	if (vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
>> +		copy_n = 1;
>> +		heads = &heads[count - 1];
> 
> 
> Do we need to check whether or not the buffer is fully used before doing
> this?
> 

This is the caller / user of __vhost_add_used_n's duty to make sure all buffer is fully used.
The device will only batch if all heads are fully used.

> 
>> +	}
>>   	start = vq->last_used_idx & (vq->num - 1);
>>   	used = vq->used->ring + start;
>> -	if (vhost_put_used(vq, heads, start, count)) {
>> +	if (vhost_put_used(vq, heads, start, copy_n)) {
>>   		vq_err(vq, "Failed to write used");
>>   		return -EFAULT;
>>   	}
>> @@ -2410,7 +2420,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct
>> vring_used_elem *heads,
>>   
>>   	start = vq->last_used_idx & (vq->num - 1);
>>   	n = vq->num - start;
>> -	if (n < count) {
>> +	if (n < count && !vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
>>   		r = __vhost_add_used_n(vq, heads, n);
>>   		if (r < 0)
>>   			return r;
>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>> index d9109107af08..0d5c49a30421 100644
>> --- a/drivers/vhost/vhost.h
>> +++ b/drivers/vhost/vhost.h
>> @@ -236,6 +236,7 @@ enum {
>>   	VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
>>   			 (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
>>   			 (1ULL << VIRTIO_RING_F_EVENT_IDX) |
>> +			 (1ULL << VIRTIO_F_IN_ORDER) |
>>   			 (1ULL << VHOST_F_LOG_ALL) |
> 
> 
> Are we sure all vhost devices can support in-order (especially the SCSI)?
> 
> It looks better to start from a device specific one.
> 
> Thanks
> 
Sorry for my mistake, I will change it.

> 
>>   			 (1ULL << VIRTIO_F_ANY_LAYOUT) |
>>   			 (1ULL << VIRTIO_F_VERSION_1)