Re: [dpdk-dev] [PATCH v4 4/4] vhost: destroy unused virtqueues when multiqueue not negotiated

[Tiwei Bie <tiwei.bie@intel.com>]

On Mon, Dec 11, 2017 at 04:15:03PM +0100, Maxime Coquelin wrote:
> QEMU sends VHOST_USER_SET_VRING_CALL requests for all queues
> declared in QEMU command line before the guest is started.
> It has the effect in DPDK vhost-user backend to allocate vrings
> for all queues declared by QEMU.
> 
> If the first driver being used does not support multiqueue,
> the device never changes to VIRTIO_DEV_RUNNING state as only
> the first queue pair is initialized. One driver impacted by
> this bug is virtio-net's iPXE driver which does not support
> VIRTIO_NET_F_MQ feature.
> 
> It is safe to destroy unused virtqueues in SET_FEATURES request
> handler, as it is ensured the device is not in running state
> at this stage, so virtqueues aren't being processed.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  lib/librte_vhost/vhost_user.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
> index 471b1612c..d5ca1ac90 100644
> --- a/lib/librte_vhost/vhost_user.c
> +++ b/lib/librte_vhost/vhost_user.c
> @@ -216,6 +216,24 @@ vhost_user_set_features(struct virtio_net *dev, uint64_t features)
>  		(dev->features & (1 << VIRTIO_NET_F_MRG_RXBUF)) ? "on" : "off",
>  		(dev->features & (1ULL << VIRTIO_F_VERSION_1)) ? "on" : "off");
>  
> +	if (!(dev->features & (1ULL << VIRTIO_NET_F_MQ))) {
> +		/*
> +		 * Remove all but first queue pair if MQ hasn't been
> +		 * negotiated. This is safe because the device is not
> +		 * running at this stage.
> +		 */
> +		while (dev->nr_vring > 2) {
> +			struct vhost_virtqueue *vq;
> +
> +			vq = dev->virtqueue[--dev->nr_vring];
> +			if (!vq)
> +				continue;
> +
> +			cleanup_vq(vq, 1);
> +			free_vq(vq);

Hi,

Sorry, I didn't look into this patch in last version.

The freed dev->virtqueue[$idx] also needs to be zeroed.
Otherwise, it won't be allocated in the future due to the
below check in vhost_user_check_and_alloc_queue_pair(),
and the freed memory will be used again.

/*
 * Allocate a queue pair if it hasn't been allocated yet
 */
static int
vhost_user_check_and_alloc_queue_pair(struct virtio_net *dev, VhostUserMsg *msg)
{
        ........

	if (dev->virtqueue[vring_idx])
		return 0;

	return alloc_vring_queue(dev, vring_idx);
}

Best regards,
Tiwei Bie

> +		}
> +	}
> +
>  	return 0;
>  }
>  
> -- 
> 2.14.3
>