Re: [Qemu-devel] [PATCH] Migration compatibility for serial

[Dr. David Alan Gilbert]

* Juan Quintela (address@hidden) wrote:
> "Michael S. Tsirkin" <address@hidden> wrote:
> > On Wed, Jun 17, 2015 at 10:11:48AM +0200, Paolo Bonzini wrote:
> >> 
> >> 
> >> On 17/06/2015 09:52, Michael S. Tsirkin wrote:
> >> > > No, please.  Upstream QEMU doesn't want to get into judgement about 
> >> > > when
> >> > > migration quality might be "good enough" that you can drop subsections.
> >> > >  It's one thing to perfect the .needed functions to make the appearance
> >> > > of subsections as unlikely as possible, but adding flags is not
> >> > > something we've done so far---and not something at least *I* want to 
> >> > > do.
> >> > 
> >> > Not like this, sure.  But e.g. patches that force specific fields to
> >> > behave in a way consistent with QEMU 2.2, with appropriate
> >> > doducmentation would be ok I think.
> >> 
> >> That's not what 2.2 means in "pc-i440fx-2.2".  It means "same hardware
> >> as 2.2", not "bug-compatible with 2.2".
> >> 
> >> Refining the .needed functions (e.g. see commit bfa7362889) is just
> >> that: describing when a subsection is needed.  Forcing specific fields
> >> to behave in a way consistent with QEMU 2.2 is bug compatibility.
> >> 
> >> Paolo
> >
> > We do bug-compatible if it's not a big pain, too.
> 
> In this case, there is disagreement about what is better:
> - correct solution
> - bug compatible
> 
> We can't have both in this case :-(
> 
> Notice that if "both" are 2.2 <improved>, i.e. 2.3 with -M
> pc-i440fx-2.2, we also got the correct behaviour.  So the matrix is
> something like:
> 
> Source: 2.2  Destination: 2.2 -> bug compatible 2.2
> Source: 2.3  Destination: 2.2 -> breaks if serial is being used, works 
> otherwise
> Source: 2.3  Destination: 2.3 with -M pc-i440fx-2.2: works always

To be fair the 2.3->2.2 is more subtle; opening it is unlikely
to generate the subsections; it needs a bit more than that (certainly on Linux)
figuring out exactly what triggers each subsection is trickier.

Dave

> 
> 
> So the problem is 2.3 -> 2.2 when serial is being used (notice that just
> opening it it is using).  That is what we are differing about what is
> the right thing to do.  As Paolo says, in upstream, we have done in the
> past the correct thing, in downstream, it depends.
> 
> Notice that adding this patch makes that the three cases are bug
> compatible, i.e. there is no way to detect breakage neither a way to fix
> the issue (fix without the patch is just upgrade both binaries.
> )
> 
> Later, Juan.
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK