Subject: Re: [PATCH net] virtio-net: Fix operstate for virtio when no VIRTIO_NET_F_STATUS

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 22 Mar 2018 16:08:28 +0200

On Thu, Mar 22, 2018 at 12:02:10PM +0000, Jay Vosburgh wrote:
> Michael S. Tsirkin <[email-masked]> wrote:
> 
> >On Thu, Mar 22, 2018 at 09:05:52AM +0000, Jay Vosburgh wrote:
> >> 	The operstate update logic will leave an interface in the
> >> default UNKNOWN operstate if the interface carrier state never changes
> >> from the default carrier up state set at creation.  This includes the
> >> case of an explicit call to netif_carrier_on, as the carrier on to on
> >> transition has no effect on operstate.
> >> 
> >> 	This affects virtio-net for the case that the virtio peer does
> >> not support VIRTIO_NET_F_STATUS (the feature that provides carrier state
> >> updates).  Without this feature, the virtio specification states that
> >> "the link should be assumed active," so, logically, the operstate should
> >> be UP instead of UNKNOWN.  This has impact on user space applications
> >> that use the operstate to make availability decisions for the interface.
> >> 
> >> 	Resolve this by changing the virtio probe logic slightly to call
> >> netif_carrier_off for both the "with" and "without" VIRTIO_NET_F_STATUS
> >> cases, and then the existing call to netif_carrier_on for the "without"
> >> case will cause an operstate transition.
> >> 
> >> Cc: "Michael S. Tsirkin" <[email-masked]>
> >> Cc: Jason Wang <[email-masked]>
> >> Cc: Ben Hutchings <[email-masked]>
> >> Fixes: 167c25e4c550 ("virtio-net: init link state correctly")
> >
> >I'd say that's an abuse of this notation. openstate was UNKNOWN
> >even before that fix.
> 
> 	I went back to the commit that added the dependency on
> VIRTIO_NET_F_STATUS (and that this patch would thus apply on top of).
> If that's an issue, I can resubmit without it.
> 
> 	-J

The patch can be trivially backported to any version that has virtio.

The issue was present since the original version of virtio.
VIRTIO_NET_F_STATUS fixed it for new devices.
So the tag is incorrectly blaming a partial fix for not being a full
one.

Also, I think it's more appropriate for net-next - it's a
minor ABI change (previously presence of VIRTIO_NET_F_STATUS
could be detected by looking at operstate, now it can't).
Hopefully this makes more apps work than it breaks.

So yes, pls repost without Fixes and with net-next unless
davem can make the change himself.

> >> Signed-off-by: Jay Vosburgh <[email-masked]>
> >
> >Acked-by: Michael S. Tsirkin <[email-masked]>
> >
> >
> >> ---
> >> 
> >> 	I considered resolving this by changing linkwatch_init_dev to
> >> unconditionally call rfc2863_policy, as that would always set operstate
> >> for all interfaces.
> >> 
> >> 	This would not have any impact on most cases (as most drivers
> >> call netif_carrier_off during probe), except for the loopback device,
> >> which currently has an operstate of UNKNOWN (because it never does any
> >> carrier state transitions).  This change would add a round trip on the
> >> dev_base_lock for every loopback device creation, which could have a
> >> negative impact when creating many loopback devices, e.g., when
> >> concurrently creating large numbers of containers.
> >> 
> >> 
> >>  drivers/net/virtio_net.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> index 23374603e4d9..7b187ec7411e 100644
> >> --- a/drivers/net/virtio_net.c
> >> +++ b/drivers/net/virtio_net.c
> >> @@ -2857,8 +2857,8 @@ static int virtnet_probe(struct virtio_device *vdev)
> >>  
> >>  	/* Assume link up if device can't report link status,
> >>  	   otherwise get link status from config. */
> >> +	netif_carrier_off(dev);
> >>  	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_STATUS)) {
> >> -		netif_carrier_off(dev);
> >>  		schedule_work(&vi->config_work);
> >>  	} else {
> >>  		vi->status = VIRTIO_NET_S_LINK_UP;
> >> -- 
> >> 2.14.1