Re: [PATCH v4 3/4] migration: zero-copy flush only at the end of bitmap scanning

[Peter Xu]

On Tue, Jun 21, 2022 at 12:35:54AM -0300, Leonardo Brás wrote:
> > This reminded me - Leo, have you considered adding the patch altogether to
> > detect the "fallback to non-zero-copy" condition?  Because when with it and
> > when the fallback happens at some point (e.g. when the guest memory is
> > larger than some value) we'll know.
> 
> I still did not consider that, but sure, how do you see that working?
> 
> We can't just disable zero-copy-send because the user actually opted in, so we
> could instead add a one time error message for when it falls back to copying, 
> as
> it should happen in the first try of zero-copy send.
> 
> Or we could fail the migration, stating the interface does not support
> MSG_ZEROCOPY, since it should happen in the first sendmsg().
> 
> I would personally opt for the last option.
> 
> What do you think?

I don't have a very strong feeling on it, but yes that sounds proper to me.
If one day we'll like zero-copy send be on by default then we'll consider
the other way round, but maybe not necessary for now.  Thanks.

-- 
Peter Xu