netdev - Re: [PATCH net-next v2 1/3] virtio_net: propagate linkspeed/duplex settings from the hypervisor

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Date:   Thu, 28 Dec 2017 10:53:54 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        qemu-devel@...gnu.org, mst@...hat.com, jasowang@...hat.com
Subject: Re: [PATCH net-next v2 1/3] virtio_net: propagate linkspeed/duplex
 settings from the hypervisor



On 12/27/2017 04:43 PM, David Miller wrote:
> From: Jason Baron <jbaron@...mai.com>
> Date: Fri, 22 Dec 2017 16:54:01 -0500
> 
>> The ability to set speed and duplex for virtio_net in useful in various
>> scenarios as described here:
>>
>> 16032be virtio_net: add ethtool support for set and get of settings
>>
>> However, it would be nice to be able to set this from the hypervisor,
>> such that virtio_net doesn't require custom guest ethtool commands.
>>
>> Introduce a new feature flag, VIRTIO_NET_F_SPEED_DUPLEX, which allows
>> the hypervisor to export a linkspeed and duplex setting. The user can
>> subsequently overwrite it later if desired via: 'ethtool -s'.
>>
>> Signed-off-by: Jason Baron <jbaron@...mai.com>
>> Cc: "Michael S. Tsirkin" <mst@...hat.com>
>> Cc: Jason Wang <jasowang@...hat.com>
> 
> Looks mostly fine to me but need some virtio_net reviewers on this one.
> 
>> @@ -57,6 +57,8 @@
>>  					 * Steering */
>>  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
>>  
>> +#define VIRTIO_NET_F_SPEED_DUPLEX 63	/* Host set linkspeed and duplex */
>> +
> 
> Why use a value so far away from the largest existing one?
> 
> Just curious.
> 

So that came from a discussion with Michael about which bit to use for
this, and he suggested using 63:

"
Transports started from bit 24 and are growing up.
So I would say devices should start from bit 63 and grow down.
"

https://patchwork.ozlabs.org/patch/848814/#1826669

I will add a comment to explain it.

Thanks,

-Jason