Re: [PATCH 2/2] vhost: only drop vqs with built-in virtio_net.c driver

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Stefan,

On 01/31/2018 06:46 PM, Stefan Hajnoczi wrote:
> Commit e29109323595beb3884da58126ebb3b878cb66f5 ("vhost: destroy unused
> virtqueues when multiqueue not negotiated") broke vhost-scsi by removing
> virtqueues when the virtio-net-specific VIRTIO_NET_F_MQ feature bit is
> missing.
> 
> The vhost_user.c code shouldn't assume all devices are vhost net device
> backends.  Use the new VIRTIO_DEV_BUILTIN_VIRTIO_NET flag to check
> whether virtio_net.c is being used.
> 
> This fixes examples/vhost_scsi.
> 
> Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>   lib/librte_vhost/vhost_user.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
> index 1dd1a61b6..65ee33919 100644
> --- a/lib/librte_vhost/vhost_user.c
> +++ b/lib/librte_vhost/vhost_user.c
> @@ -187,7 +187,8 @@ 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))) {
> +	if ((dev->flags & VIRTIO_DEV_BUILTIN_VIRTIO_NET) &&
> +	    !(dev->features & (1ULL << VIRTIO_NET_F_MQ))) {

If we had an external net backend using the library, we would also need 
to remove extra queues if it advertised VIRTIO_NET_F_MQ, but the feature
isn't negotiated.

In this case, the fix I suggested yesterday would work:
if ((vhost_features & (1ULL << VIRTIO_NET_F_MQ)) &&
     !(dev->features & (1ULL << VIRTIO_NET_F_MQ)) {
...
}

For any backend that does not advertise the feature, no queues will be
destroyed.

Thanks,
Maxime
>   		/*
>   		 * Remove all but first queue pair if MQ hasn't been
>   		 * negotiated. This is safe because the device is not
>