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

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

On Mon, Nov 14, 2022 at 11:35:30PM +0800, Hyman wrote:
> 
> 
> 在 2022/11/11 3:17, Michael S. Tsirkin 写道:
> > 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?
> When openvswitch restart and reconnect and Qemu start the vhost_dev，
> acked_features in vhost_dev Qemu need to be initialized and the initialized
> value be fetched from acked_features int NetVhostUserState.
> At this time, acked_features  may not be up-to-date but we want it.
> > 
> > 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?
> > 
> I'm trying to explain this from my view, please point out the mistake if i
> failed. :)
> 
> When socket used by vhost-user device disconnectted from openvswitch,
> Qemu will stop the vhost-user and clean up the whole struct of
> vhost_dev(include vm's memory region and acked_features), once socket is
> reconnected from openvswitch, Qemu will collect vm's memory region
> dynamically but as to acked_features, IMHO, Qemu can not fetch it from guest
> features of virtio-net, because acked_features are kind of different from
> guest features(bit 30 is different at least)，so Qemu need an extra copy.

Well we already have a list of valid frontend features in
user_feature_bits.



> > 
> > 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
> >