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

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Date:   Thu, 22 Mar 2018 12:02:10 +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 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

>> 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