Re: [dpdk-dev] [PATCH v3 1/4] vhost: prevent features to be changed while device is running

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

On Wed, Dec 06, 2017 at 10:20:45AM +0100, Maxime Coquelin wrote:
> As section 2.2 of the Virtio spec states about features
> negotiation:
> "During device initialization, the driver reads this and tells
> the device the subset that it accepts. The only way to
> renegotiate is to reset the device."
> 
> This patch implements a check to prevent illegal features change
> while the device is running.
> 
> One exception is the VHOST_F_LOG_ALL feature bit, which is enabled
> when live-migration is initiated. But this feature is not negotiated
> with the Virtio driver, but directly with the Vhost master.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  lib/librte_vhost/vhost_user.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
> index f4c7ce462..2d86c0ca8 100644
> --- a/lib/librte_vhost/vhost_user.c
> +++ b/lib/librte_vhost/vhost_user.c
> @@ -183,7 +183,22 @@ vhost_user_set_features(struct virtio_net *dev, uint64_t features)
>  		return -1;
>  	}
>  
> -	if ((dev->flags & VIRTIO_DEV_RUNNING) && dev->features != features) {
> +	if (dev->features == features)
> +		return 0;
> +

We couldn't return directly when dev->features == features.
Otherwise, if the features provided by virtio driver is 0,
dev->vhost_hlen won't get a chance to be initialized.

Best regards,
Tiwei Bie