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" <m...@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" <m...@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
