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

[Michael S. Tsirkin]

On Wed, Jun 17, 2015 at 12:48:00PM +0200, Juan Quintela 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 :-(

I think this depends on how major and/or common the bug is.

> 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
> 
> 
> 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.

I would say it's the "correct" thing. We don't really know for sure
it's correct, we think so.  We might in theory have bugs in 2.4 code
too.  Whether we are better off failing migration depends on how severe
the bugs are.

-- 
MST