netdev - Re: [PATCH, net-next, v3, 0/2] net/ethtool: Introduce link_ksettings API for virtual network devices

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Date:   Fri, 20 Dec 2019 14:30:01 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Miller <davem@...emloft.net>
Cc:     cforno12@...ux.vnet.ibm.com, netdev@...r.kernel.org,
        mst@...hat.com, jasowang@...hat.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, sashal@...nel.org, tlfalcon@...ux.ibm.com
Subject: Re: [PATCH, net-next, v3, 0/2] net/ethtool: Introduce
 link_ksettings API for virtual network devices

On Thu, 19 Dec 2019 14:16:19 -0800 (PST)
David Miller <davem@...emloft.net> wrote:

> From: Stephen Hemminger <stephen@...workplumber.org>
> Date: Thu, 19 Dec 2019 13:11:56 -0800
> 
> > I don't think this makes sense for netvsc. The speed and duplex have no
> > meaning, why do you want to allow overriding it? If this is to try and make
> > some dashboard look good; then you aren't seeing the real speed which is
> > what only the host knows. Plus it does take into account the accelerated
> > networking path.  
> 
> Maybe that's the point, userspace has extraneous knowledge it might
> use to set it accurately.
> 
> This helps for bonding/team etc. as well.
> 
> I don't think there is any real harm in allowing to set this, and
> we've done this in the past I think.

Looked a little more. The netvsc driver does query the host already
and get a correct link speed.  See drivers/net/hyperv/rndis_filter.c

If running on Windows Server 2019 you will see speed report of 40G
if using 40G Mellanox SRIOV, and 10G if using a private virtual network.

On Azure, the guest sees the speed of the underlying network connection
which varies based on machine type.

Bottom line: it behaves like real hardware now, why should we allow user
to set it for netvsc and not other real hardware.