Re: [RFC v3 1/7] vhost: expose used buffers

[Guo Zhi]

----- Original Message -----
> From: "jasowang" <jasowang@xxxxxxxxxx>
> To: "Guo Zhi" <qtxuning1999@xxxxxxxxxxx>, "eperezma" <eperezma@xxxxxxxxxx>, "sgarzare" <sgarzare@xxxxxxxxxx>, "Michael
> Tsirkin" <mst@xxxxxxxxxx>
> Cc: "netdev" <netdev@xxxxxxxxxxxxxxx>, "linux-kernel" <linux-kernel@xxxxxxxxxxxxxxx>, "kvm list" <kvm@xxxxxxxxxxxxxxx>,
> "virtualization" <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>
> Sent: Wednesday, September 7, 2022 12:21:06 PM
> Subject: Re: [RFC v3 1/7] vhost: expose used buffers

> 在 2022/9/1 13:54, 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@xxxxxxxxxxx>
>> ---
>>   drivers/vhost/vhost.c | 16 +++++++++++++---
>>   1 file changed, 13 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 40097826cff0..26862c8bf751 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];
>> +	}
> 
> 
> Would it better to have a dedicated helper like
> vhost_add_used_in_order() here?
> 
That would be much more convenient and clear to implement. 
I think have a dedicated function for in order feature in vhost is better.

> 
> 
>>   	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;
>>   	}
>> @@ -2388,7 +2398,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>>   		smp_wmb();
>>   		/* Log used ring entry write. */
>>   		log_used(vq, ((void __user *)used - (void __user *)vq->used),
>> -			 count * sizeof *used);
>> +			 copy_n * sizeof(*used));
>>   	}
>>   	old = vq->last_used_idx;
>>   	new = (vq->last_used_idx += count);
>> @@ -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)) {
> 
> 
> This seems strange, any reason for this? (Actually if we support
> in-order we only need one used slot which fit for the case here)
> 
> Thanks
> 
If in order feature negotiated, even the count is larger than n,
we don't need to call __vhost_add_used_n again, because in order 
only use one slot.

Thanks
> 
>>   		r = __vhost_add_used_n(vq, heads, n);
>>   		if (r < 0)
>>   			return r;