Re: [PATCH v3 4/6] vhost-user: Add queue pair parameter throughout the network stack

[Laurent Vivier <lvivier@redhat.com>]

On 12/11/25 08:01, Stefano Brivio wrote:
> On Wed,  3 Dec 2025 19:54:32 +0100
> Laurent Vivier <lvivier@redhat.com> wrote:
> 
>> diff --git a/vu_common.c b/vu_common.c
>> index b13b7c308fd8..80d9a30f6f71 100644
>> --- a/vu_common.c
>> +++ b/vu_common.c
>> @@ -196,11 +196,11 @@ static void vu_handle_tx(struct vu_dev *vdev, int index,
>>   
>>   		data = IOV_TAIL(elem[count].out_sg, elem[count].out_num, 0);
>>   		if (IOV_DROP_HEADER(&data, struct virtio_net_hdr_mrg_rxbuf))
>> -			tap_add_packet(vdev->context, &data, now);
>> +			tap_add_packet(vdev->context, 0, &data, now);
>>   
>>   		count++;
>>   	}
>> -	tap_handler(vdev->context, now);
>> +	tap_handler(vdev->context, 0, now);
>>   
>>   	if (count) {
>>   		int i;
>> @@ -235,23 +235,26 @@ void vu_kick_cb(struct vu_dev *vdev, union epoll_ref ref,
>>   }
>>   
>>   /**
>> - * vu_send_single() - Send a buffer to the front-end using the RX virtqueue
>> + * vu_send_single() - Send a buffer to the front-end using a specified virtqueue
>>    * @c:		execution context
>> + * @qpair:	Queue pair on which to send the buffer
>>    * @buf:	address of the buffer
>>    * @size:	size of the buffer
>>    *
>>    * Return: number of bytes sent, -1 if there is an error
>>    */
>> -int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>> +int vu_send_single(const struct ctx *c, unsigned int qpair, const void *buf, size_t size)
>>   {
>>   	struct vu_dev *vdev = c->vdev;
>> -	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
>>   	struct vu_virtq_element elem[VIRTQUEUE_MAX_SIZE];
>>   	struct iovec in_sg[VIRTQUEUE_MAX_SIZE];
>> +	struct vu_virtq *vq;
>>   	size_t total;
>>   	int elem_cnt;
>>   	int i;
>>   
>> +	vq = &vdev->vq[qpair << 1];
> 
> << 1 instead of * 2 is a bit surprising here, for a few seconds I
> thought you swapped qpair and 1.
> 
> Then I started thinking that somebody is likely to mix up (probably not
> you) indices of RX and TX queues at some point. So... what about some
> macros, say (let's see if I got it right this time):
> 
> #define VHOST_SEND_QUEUE(pair)	((pair) * 2)
> #define VHOST_RECV_QUEUE(pair)	(pair)

I will. David had the same comment. TX and RX are from the point of view of guest, it's 
not obvious when we read passt code.

I would prefer as David proposed to use, i.e. FROMGUEST and TOGUEST:

#define VHOST_FROM_GUEST(qpair) ((qpair) * 2 + 1)
#define VHOST_TO_GUEST(qpair)   ((qpair) * 2)>
> and:
> 
> #define VHOST_QUEUE_PAIR(q)	((q) % 2) ? (q) : (q) / 2)

I don't undestand the purpose of this one.

> 
> ...are they correct? A short description or "Theory of operation"
> section somewhere with a recap of how queue indices are used would be
> nice to have.
> 
> And maybe also something explaining that 0 that's now appearing in
> argument lists:
> 
> #define VHOST_NO_QUEUE		0

It's not really NO_QUEUE, it's default queue pair, the queue pair 0

Thanks,
Laurent