Re: [RFC] virtio: enforce link up

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

On Sat, Oct 14, 2023 at 09:06:09PM +0000, Vincent Jardin wrote:
> On 10/14/23 18:37, Michael S. Tsirkin wrote:
> > On Sat, Oct 14, 2023 at 06:22:34PM +0200, Vincent Jardin wrote:
> >> Using interface's settings, let's enforce an always on link up.
> >>
> >> Signed-off-by: Vincent Jardin <vincent.jardin@ekinops.com>
> > 
> > What is going on here? Just don't set it down.
> 
> The purpose is to have a stable vLink for the VMs that don't support 
> such sysctl arp_evict_nocarrier:
> https://patchwork.kernel.org/project/netdevbpf/patch/20211101173630.300969-2-prestwoj@gmail.com/
> 
> We are facing some users of vSwitches that use vhost-user and that 
> disconnect and reconnect during some operations. For most of the VMs on 
> their deployments with such vSwitches, those VMs' vLink should not flap.
> 
> For those VMs, the flaps are critical and they can lead to some 
> convergence issues.
> 
> If this capability is not at the virtio-net level, should it be at 
> qemu's net_vhost_user_event() ?
> For instance, from 
> https://github.com/qemu/qemu/blob/63011373ad22c794a013da69663c03f1297a5c56/net/vhost-user.c#L266 
> ?
> 
> best regards,
>    Vincent


makes sense

> 
> > 
> >> ---
> >>   hw/net/virtio-net.c            | 8 ++++++++
> >>   include/hw/virtio/virtio-net.h | 2 ++
> >>   2 files changed, 10 insertions(+)
> >>
> >> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> >> index 29e33ea5ed..e731b4fdea 100644
> >> --- a/hw/net/virtio-net.c
> >> +++ b/hw/net/virtio-net.c
> >> @@ -78,6 +78,9 @@
> >>      tso/gso/gro 'off'. */
> >>   #define VIRTIO_NET_RSC_DEFAULT_INTERVAL 300000
> >>   
> >> +/* force always link up */
> >> +#define VIRTIO_NET_LINK_UP false
> >> +
> >>   #define VIRTIO_NET_RSS_SUPPORTED_HASHES (VIRTIO_NET_RSS_HASH_TYPE_IPv4 | \
> >>                                            VIRTIO_NET_RSS_HASH_TYPE_TCPv4 | \
> >>                                            VIRTIO_NET_RSS_HASH_TYPE_UDPv4 | \
> >> @@ -447,6 +450,9 @@ static void virtio_net_set_link_status(NetClientState *nc)
> >>       else
> >>           n->status |= VIRTIO_NET_S_LINK_UP;
> >>   
> >> +    if (n->net_conf.link_up)
> >> +        n->status |= VIRTIO_NET_S_LINK_UP;
> >> +
> >>       if (n->status != old_status)
> >>           virtio_notify_config(vdev);
> >>   
> >> @@ -3947,6 +3953,8 @@ static Property virtio_net_properties[] = {
> >>                         VIRTIO_NET_F_GUEST_USO6, true),
> >>       DEFINE_PROP_BIT64("host_uso", VirtIONet, host_features,
> >>                         VIRTIO_NET_F_HOST_USO, true),
> >> +    DEFINE_PROP_BOOL("link_up", VirtIONet, net_conf.link_up,
> >> +                       VIRTIO_NET_LINK_UP),
> >>       DEFINE_PROP_END_OF_LIST(),
> >>   };
> >>   
> >> diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> >> index 55977f01f0..385bebab34 100644
> >> --- a/include/hw/virtio/virtio-net.h
> >> +++ b/include/hw/virtio/virtio-net.h
> >> @@ -56,6 +56,7 @@ typedef struct virtio_net_conf
> >>       char *duplex_str;
> >>       uint8_t duplex;
> >>       char *primary_id_str;
> >> +    bool link_up; /* if set enforce link up, never down */
> >>   } virtio_net_conf;
> >>   
> >>   /* Coalesced packets type & status */
> >> @@ -180,6 +181,7 @@ struct VirtIONet {
> >>       size_t guest_hdr_len;
> >>       uint64_t host_features;
> >>       uint32_t rsc_timeout;
> >> +    uint32_t link_up; /* if set enforce link up, never down */
> >>       uint8_t rsc4_enabled;
> >>       uint8_t rsc6_enabled;
> >>       uint8_t has_ufo;
> >> -- 
> >> 2.34.1
> > 
>