Re: [PATCH v3 2/6] vhost-user: Enable multiqueue

[Stefano Brivio <sbrivio@redhat.com>]

On Wed,  3 Dec 2025 19:54:30 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> Advertise multi-queue support in vhost-user by setting VIRTIO_NET_F_MQ
> and VHOST_USER_PROTOCOL_F_MQ feature flags, and increase
> VHOST_USER_MAX_VQS from 2 to 32, supporting up to 16 queue pairs.
> 
> Currently, only the first RX queue (queue 0) is used for receiving
> packets. The guest kernel selects which TX queue to use for
> transmission. Full multi-RX queue load balancing will be implemented in
> future work.
> 
> Update the QEMU usage hint to show the required parameters for enabling
> multiqueue: queues parameter on the netdev, and mq=true on the
> virtio-net device.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  tap.c        |  7 +++++--
>  vhost_user.c | 10 ++++++----
>  virtio.h     |  2 +-
>  3 files changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/tap.c b/tap.c
> index 2cda8c9772b8..591b49491aa3 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -1314,8 +1314,11 @@ static void tap_backend_show_hints(struct ctx *c)
>  		break;
>  	case MODE_VU:
>  		info("You can start qemu with:");
> -		info("    kvm ... -chardev socket,id=chr0,path=%s -netdev vhost-user,id=netdev0,chardev=chr0 -device virtio-net,netdev=netdev0 -object memory-backend-memfd,id=memfd0,share=on,size=$RAMSIZE -numa node,memdev=memfd0\n",
> -		     c->sock_path);
> +		info("    kvm ... -chardev socket,id=chr0,path=%s "
> +		     "-netdev vhost-user,id=netdev0,chardev=chr0,queues=$QUEUES "
> +		     "-device virtio-net,netdev=netdev0,mq=true "
> +		     "-object memory-backend-memfd,id=memfd0,share=on,size=$RAMSIZE "
> +		     "-numa node,memdev=memfd0\n", c->sock_path);
>  		break;
>  	}
>  }
> diff --git a/vhost_user.c b/vhost_user.c
> index aa7c869d9e56..845fdb551c84 100644
> --- a/vhost_user.c
> +++ b/vhost_user.c
> @@ -323,6 +323,7 @@ static bool vu_get_features_exec(struct vu_dev *vdev,
>  	uint64_t features =
>  		1ULL << VIRTIO_F_VERSION_1 |
>  		1ULL << VIRTIO_NET_F_MRG_RXBUF |
> +		1ULL << VIRTIO_NET_F_MQ |
>  		1ULL << VHOST_F_LOG_ALL |
>  		1ULL << VHOST_USER_F_PROTOCOL_FEATURES;
>  
> @@ -767,7 +768,8 @@ static void vu_check_queue_msg_file(struct vhost_user_msg *vmsg)
>  	int idx = vmsg->payload.u64 & VHOST_USER_VRING_IDX_MASK;
>  
>  	if (idx >= VHOST_USER_MAX_VQS)
> -		die("Invalid vhost-user queue index: %u", idx);
> +		die("Invalid vhost-user queue index: %u (maximum %u)", idx,
> +		    VHOST_USER_MAX_VQS);
>  
>  	if (nofd) {
>  		vmsg_close_fds(vmsg);
> @@ -896,7 +898,8 @@ static bool vu_get_protocol_features_exec(struct vu_dev *vdev,
>  	uint64_t features = 1ULL << VHOST_USER_PROTOCOL_F_REPLY_ACK |
>  			    1ULL << VHOST_USER_PROTOCOL_F_LOG_SHMFD |
>  			    1ULL << VHOST_USER_PROTOCOL_F_DEVICE_STATE |
> -			    1ULL << VHOST_USER_PROTOCOL_F_RARP;
> +			    1ULL << VHOST_USER_PROTOCOL_F_RARP |
> +			    1ULL << VHOST_USER_PROTOCOL_F_MQ;
>  
>  	(void)vdev;
>  	vmsg_set_reply_u64(vmsg, features);
> @@ -935,10 +938,9 @@ static bool vu_get_queue_num_exec(struct vu_dev *vdev,
>  {
>  	(void)vdev;
>  
> -	/* NOLINTNEXTLINE(misc-redundant-expression) */
>  	vmsg_set_reply_u64(vmsg, VHOST_USER_MAX_VQS / 2);
>  
> -	debug("VHOST_USER_MAX_VQS  %u", VHOST_USER_MAX_VQS / 2);
> +	debug("queue num  %u", VHOST_USER_MAX_VQS / 2);

Nit, if you respin: this "queue num  %u" message doesn't carry any
context at all. Actually, why is it needed? It's defined at build time
anyway.

If it's needed maybe "Using up to %u vhost-user queue pairs"?

>  
>  	return true;
>  }
> diff --git a/virtio.h b/virtio.h
> index 12caaa0b6def..176c935cecc7 100644
> --- a/virtio.h
> +++ b/virtio.h
> @@ -88,7 +88,7 @@ struct vu_dev_region {
>  	uint64_t mmap_addr;
>  };
>  
> -#define VHOST_USER_MAX_VQS 2
> +#define VHOST_USER_MAX_VQS 32
>  
>  /*
>   * Set a reasonable maximum number of ram slots, which will be supported by

-- 
Stefano