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

11 Dec 2025

On 12/11/25 13:16, Stefano Brivio wrote:
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On Thu, 11 Dec 2025 09:48:42 +0100
Laurent Vivier  wrote:
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On 12/11/25 08:01, Stefano Brivio wrote:
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On Wed,  3 Dec 2025 19:54:32 +0100
Laurent Vivier  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.
Uh, wait, I must have missed it. Do you have a Message-ID? I'm afraid I
must have missed some emails here but I don't see them in archives
either...
Message-ID: aRF1_Qj6uxf1ndiA@zatzit
...
...
TX and RX are from the point of view of guest, it's
not obvious when we read passt code.
Right, yes, for me neither, I always get confused. That's why I thought
we could make the RX vhost-user queue become "SEND" in passt's code,
but:
...
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)
...this is even clearer. It misses the QUEUE though. Does
VHOST_QUEUE_{FROM,TO}_GUEST fit where you use it? Otherwise I guess VQ
together with FROM / TO should be clear enough.
...
...
and:
#define VHOST_QUEUE_PAIR(q)	((q) % 2) ? (q) : (q) / 2)
I don't undestand the purpose of this one.
To get the pair number from a queue number. You're doing something like
that (I guess?) in 5/6, vu_handle_tx():
+	tap_flush_pools(index / 2);
+			tap_add_packet(vdev->context, index / 2, &data, now);
+	tap_handler(vdev->context, index / 2, now);
but now that I see your definition for VHOST_FROM_GUEST() above, and
that the purpose wasn't clear to you, I guess it should be:
#define VHOST_PAIR_FROM_QUEUE(q) (((q) % 2) ? ((q) - 1 / 2) : ((q) / 2))
Why not simply:

#define VHOST_PAIR_FROM_QUEUE(q) (q / 2)

QUEUES 0,1 -> QP 0
QUEUES 2,3 -> QP 1
...
...or maybe it's not needed? I'm not sure.
...
...
...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
Hmm but for non-vhost-user usages then it's not a queue, right? Well,
For non vhost usage we can say there is only one queue.
...
whatever, as long as we have a definition for it... or maybe we could
have VHOST_QUEUE_DEFAULT and NO_VHOST_QUEUE or VHOST_NO_QUEUE all being
0?
Perhaps we could instead use a generic naming:

QPAIR_DEFAULT
QUEUE_FROM_GUEST(qpair)
QUEUE_TO_GUEST(qpair)

Thanks,
Laurent

    