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Re: [Qemu-devel] [PATCH] net: Fix link state inter-dependencies between
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH] net: Fix link state inter-dependencies between NIC and backend |
Date: |
Thu, 2 Apr 2015 10:21:03 +0100 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Thu, Apr 02, 2015 at 08:11:15AM +0200, Michael S. Tsirkin wrote:
> On Wed, Apr 01, 2015 at 07:55:38PM -0400, Vladislav Yasevich wrote:
> > Commit 02d38fcb2caa4454cf4ed728d5908c3cc9ba47be
> > net: Update netdev peer on link change
> >
> > introduced a link state dependency between the backend
> > netdev and the nic. If the admin turned off the link
> > on the backend, the nic link state was set to down because
> > the link had no access to the network at all. This created
> > some inconsitet behavior for someone who wanted to play
> > around the links states.
> > 1) Turning off the nic link and then turning on the backend
> > link (even if it was already on) would turn on the nic link
> > again.
> > 2) Turning off the backend link and then turning on the nic
> > link would turn on the link in the VM, but would not change
> > the backend state resulting in a broken/unreachable network.
> >
> > This patch attempts to correct these behaviors. The patch treats
> > the nic-backend relationship as two ends of a wire. Each end tracks
> > the administratively set link state in addition to actual link
> > state. Thus is is possible to set link down at each end effectively
> > emulating plugging/unplugging the wire at either end. The patch
> > continues to preserve the old behavior where setting just
> > nic side down does NOT impact the backend.
> >
> > Signed-off-by: Vladislav Yasevich <address@hidden>
>
> I never understood the point of
> 02d38fcb2caa4454cf4ed728d5908c3cc9ba47be.
>
> If you want to tell guest link is down,
> just set the nic link down.
>
> How about we just revert it?
>
> Then one can change link state on each end
> independently, with no notification,
> and no state to track.
I agree. The simple solution would be nice, unless you are aware of a
use case where it causes problems.
Stefan
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