netdev - Re: [PATCH net] virtio-net: Fix operstate for virtio when no VIRTIO_NET_F

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Date:   Thu, 22 Mar 2018 14:34:40 +0000
From:   Jay Vosburgh <jay.vosburgh@...onical.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
cc:     netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        David Miller <davem@...emloft.net>,
        Ben Hutchings <ben@...adent.org.uk>
Subject: Re: [PATCH net] virtio-net: Fix operstate for virtio when no VIRTIO_NET_F_STATUS

Michael S. Tsirkin <mst@...hat.com> wrote:

>On Thu, Mar 22, 2018 at 12:02:10PM +0000, Jay Vosburgh wrote:
>> Michael S. Tsirkin <mst@...hat.com> 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" <mst@...hat.com>
>> >> Cc: Jason Wang <jasowang@...hat.com>
>> >> Cc: Ben Hutchings <ben@...adent.org.uk>
>> >> 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.

	Reposting with requested changes.

	-J

>> >> Signed-off-by: Jay Vosburgh <jay.vosburgh@...onical.com>
>> >
>> >Acked-by: Michael S. Tsirkin <mst@...hat.com>
>> >
>> >
>> >> ---
>> >> 
>> >> 	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