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

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

On Thu, Feb 20, 2025 at 09:00:04PM +0100, Stefano Brivio wrote:
> 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.

I'm going by memory. We made it a warning on the assumption that hey,
maybe someone has a way to migrate without a RARP, let them work.
Exactly what happened, we just did not think it through completely :)


-- 
MST