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

[Michael S. Tsirkin]

On Wed, Jun 17, 2015 at 01:33:26PM +0200, Paolo Bonzini wrote:
> 
> 
> On 17/06/2015 12:13, Michael S. Tsirkin 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.
> > 
> > We do bug-compatible if it's not a big pain, too.
> 
> Where, in the specific case of migration?

Just look at hour compat flags.

For example (intentionally using serial here):
        {\
            .driver   = "pci-serial",\
            .property = "prog_if",\
            .value    = stringify(0),\
        },\
        {\
            .driver   = "pci-serial-2x",\
            .property = "prog_if",\
            .value    = stringify(0),\
        },\
        {\
            .driver   = "pci-serial-4x",\
            .property = "prog_if",\
            .value    = stringify(0),\
        },\

apparently some clients check the specific prog if
value, completely breaking if the value was incorrect.
So we fixed it for new machine types but not old ones.

We have tons of other examples for other devices.

> Like Juan, I see where you're coming from.  But it's a slippery slope,
> and upstream chose not to go down it.
> 
> Paolo

We did very similar things in the past. A minor bug is not worth
failing migration for. Is this a minor bug? Still need to
figure this out, commit logs should help more in this respect.

-- 
MST