Re: [PATCH v3] examples/vhost: workaround qemu abort

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

On Wed, Jul 25, 2018 at 06:25:56PM +0800, Marvin Liu wrote:
> Current qemu vhost net ring start has a dependency on feature bit
> VHOST_USER_F_PROTOCOL_FEATURES. Without this feature, vhost ring can't
> enabled and ioevent fd won't be deleted after vhost device stop. That
> will cause qemu abort when reloading driver. Work around qemu issues by
> enabling feature bit in vhost user backend.
> 
> Signed-off-by: Marvin Liu <yong.liu@intel.com>
> 
> diff --git a/examples/vhost/main.c b/examples/vhost/main.c
> index 2175c1186..8573004dd 100644
> --- a/examples/vhost/main.c
> +++ b/examples/vhost/main.c
> @@ -1520,7 +1520,10 @@ main(int argc, char *argv[])
>  		}
>  
>  		if (builtin_net_driver)
> -			rte_vhost_driver_set_features(file, VIRTIO_NET_FEATURES);
> +			/* Workaround for qemu vhost net device startup */
> +			rte_vhost_driver_set_features(file,
> +				VIRTIO_NET_FEATURES |
> +				1ULL << VHOST_USER_F_PROTOCOL_FEATURES);

I have taken a closer look. I think it's not really right
to just offer this bit like this. Because once this bit is
negotiated, QEMU will ask the builtin_net_driver for the
protocol features it supports. Currently, there is no API
to allow external backends like the builtin_net_driver in
this example to set the protocol features it supports, and
instead, the protocol features VHOST_USER_PROTOCOL_FEATURES
defined in librte_vhost will be returned to QEMU. This will
cause problems because the builtin_net_driver doesn't really
support those protocol features.


>  
>  		if (mergeable == 0) {
>  			rte_vhost_driver_disable_features(file,
> -- 
> 2.17.0
>