Re: [PATCH v3 2/2] vhost-net: Fix the virtio features negotiation flaw

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Oct 30, 2022 at 09:52:39PM +0800, huangy81@chinatelecom.cn wrote:
> From: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> 
> Save the acked_features once it be configured by guest
> virtio driver so it can't miss any features.
> 
> Note that this patch also change the features saving logic
> in chr_closed_bh, which originally backup features no matter
> whether the features are 0 or not, but now do it only if
> features aren't 0.
> 
> As to reset acked_features to 0 if needed, Qemu always
> keeping the backup acked_features up-to-date, and save the
> acked_features after virtio_net_set_features in advance,
> including reset acked_features to 0, so the behavior is
> also covered.
> 
> Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> Signed-off-by: Guoyi Tu <tugy@chinatelecom.cn>
> ---
>  hw/net/vhost_net.c      | 9 +++++++++
>  hw/net/virtio-net.c     | 5 +++++
>  include/net/vhost_net.h | 2 ++
>  net/vhost-user.c        | 6 +-----
>  4 files changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index d28f8b9..2bffc27 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -141,6 +141,15 @@ uint64_t vhost_net_get_acked_features(VHostNetState *net)
>      return net->dev.acked_features;
>  }
>  
> +void vhost_net_save_acked_features(NetClientState *nc)
> +{
> +    if (nc->info->type != NET_CLIENT_DRIVER_VHOST_USER) {
> +        return;
> +    }
> +
> +    vhost_user_save_acked_features(nc, false);
> +}
> +
>  static int vhost_net_get_fd(NetClientState *backend)
>  {
>      switch (backend->info->type) {
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index e9f696b..5f8f788 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -924,6 +924,11 @@ static void virtio_net_set_features(VirtIODevice *vdev, uint64_t features)
>              continue;
>          }
>          vhost_net_ack_features(get_vhost_net(nc->peer), features);
> +        /*
> +         * keep acked_features in NetVhostUserState up-to-date so it
> +         * can't miss any features configured by guest virtio driver.
> +         */
> +        vhost_net_save_acked_features(nc->peer);
>      }
>  
>      if (virtio_has_feature(features, VIRTIO_NET_F_CTRL_VLAN)) {

So when do you want to ack features but *not* save them?

Is the effect of this patch, fundamentally, that guest features
from virtio are always copied to vhost-user?
Do we even need an extra copy in vhost user then?


all this came in with:

commit a463215b087c41d7ca94e51aa347cde523831873
Author: Marc-André Lureau <marcandre.lureau@redhat.com>
Date:   Mon Jun 6 18:45:05 2016 +0200

    vhost-net: save & restore vhost-user acked features

Marc-André do you remember why we have a copy of features in vhost-user
and not just reuse the features from virtio?


> diff --git a/include/net/vhost_net.h b/include/net/vhost_net.h
> index 387e913..3a5579b 100644
> --- a/include/net/vhost_net.h
> +++ b/include/net/vhost_net.h
> @@ -46,6 +46,8 @@ int vhost_set_vring_enable(NetClientState * nc, int enable);
>  
>  uint64_t vhost_net_get_acked_features(VHostNetState *net);
>  
> +void vhost_net_save_acked_features(NetClientState *nc);
> +
>  int vhost_net_set_mtu(struct vhost_net *net, uint16_t mtu);
>  
>  #endif
> diff --git a/net/vhost-user.c b/net/vhost-user.c
> index 74f349c..c512cc9 100644
> --- a/net/vhost-user.c
> +++ b/net/vhost-user.c
> @@ -258,11 +258,7 @@ static void chr_closed_bh(void *opaque)
>      s = DO_UPCAST(NetVhostUserState, nc, ncs[0]);
>  
>      for (i = queues -1; i >= 0; i--) {
> -        s = DO_UPCAST(NetVhostUserState, nc, ncs[i]);
> -
> -        if (s->vhost_net) {
> -            s->acked_features = vhost_net_get_acked_features(s->vhost_net);
> -        }
> +        vhost_user_save_acked_features(ncs[i], false);
>      }
>  
>      qmp_set_link(name, false, &err);
> -- 
> 1.8.3.1