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

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 20 Feb 2025 13:21:33 -0500
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> 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.

Thanks for the explanation. I'm struggling to grasp this. So if a
back-end doesn't support migration, because VHOST_USER_PROTOCOL_F_RARP
is not present in the features flag, migration is done anyway, but then
this is printed:

  Vhost user backend fails to broadcast fake RARP

with the meaning of:

  We did migration even if the back-end doesn't support it, whoops

?

Note that the message is printed *after* the migration and the flag is
*not* checked before.

> If you want a new flag "migration with no RARP", be my
> guest and add it.

That would actually make more sense than the existing situation I
think. VHOST_USER_PROTOCOL_F_NO_RARP?

I didn't understand, yet, what the exact meaning would be, though.

> Or if you want to add documentation explaining the meaning
> better and clarifying the message.

I'm still in the phase where I'm trying to understand the role of the
message :) ...I have to say this is fairly different now from what was
mentioned on the thread so far.

-- 
Stefano