Re: [PATCH] vhost-user: Silence unsupported VHOST_USER_PROTOCOL_F_RARP error

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 20, 2025 at 05:59:10PM +0100, Stefano Brivio wrote:
> On Thu, 20 Feb 2025 10:28:20 -0500
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Fri, Jan 24, 2025 at 05:03:27PM +0100, Stefano Brivio wrote:
> > > But I don't understand why we're leaving this as it is.  
> > 
> > So that people notice if there's some backend problem and
> > announcements are not going out. should help debug migration
> > issues. which we had, so we added this :)
> 
> The message mentions that the back-end fails to do something it didn't
> and can't even do, that's (one reason) why it's wrong (and confusing)
> and this patch is obviously correct.
> 
> Perhaps the commit title isn't entirely accurate (it should say "when
> unsupported", I guess) but it's somewhat expected to sacrifice detail
> in the name of brevity, there. A glimpse at the message is enough.
> 
> Laurent now added a workaround in passt to pretend that we support
> VHOST_USER_PROTOCOL_F_RARP by doing nothing in the callback, report
> success, and silence the warning:
> 
>   https://passt.top/passt/commit/?id=dd6a6854c73a09c4091c1776ee7f349d1e1f966c
> 
> but having to do this kind of stuff is a bit unexpected while
> interacting with another opensource project.
> 
> -- 
> Stefano


let me explain. historically backends did not support migration.
then migration was added. as it was assumed RARP is required,
we did not add a feature flag for "supports migration" and
instead just assumed that VHOST_USER_PROTOCOL_F_RARP is that.

If you silence the warning you silence it for old backends
with no migration support.
If you want a new flag "migration with no RARP", be my
guest and add it.
Or if you want to add documentation explaining the meaning
better and clarifying the message.

-- 
MST