Re: [Qemu-devel] [Qemu-ppc] [PATCH] spapr: skip configuration section during migration of older machines

[David Gibson]

On Thu, Feb 11, 2016 at 04:53:40PM +0000, Dr. David Alan Gilbert wrote:
> * Greg Kurz (address@hidden) wrote:
> > On Mon, 08 Feb 2016 16:59:47 +0100
> > Greg Kurz <address@hidden> wrote:
> > > Since QEMU 2.4, we have a configuration section in the migration stream.
> > > This must be skipped for older machines, like it is already done for x86.
> > > 
> > 
> > Ouch ! It is more complex than I thought... the migration of pseries-2.3
> > machine is already broken between QEMU-2.3 and QEMU-2.4. So this patch
> > fixes indeed migration of a pseries-2.3 machine from QEMU-2.3, but it
> > breaks migration of the same machine from QEMU-2.4 and up.
> > 
> > Not sure how to deal with that... is it reasonable to assume that
> > pseries-2.3 running with QEMU-2.3 is the common case ? If so, this
> > patch would bring more help than harm.
> 
> Unfortunately we can not fix history, so we have to pick something to fix.
> So unless there is another reason, then I normally say keep it working
> between the latest versions of qemu; i.e. if someone is running qemu 2.5 with
> -M 2.3  then dont break it when they try and migrate to 2.6, even though
> this would fix an older qemu migrating into 2.6.

Yeah, I tend to agree, but I'd change my mind if there's evidence that
the older qemu is much more widely deployed.

IIUC that would entail no actual change to the code yes?  But I think
we should put a comment there saying what the fix would be to talk to
the older qemu, and why we chose not to apply it.

> However, as discussed on irc you might be able to fudge it; for example
> using qemu_peek_byte to test whether or not you have a configuration
> section, and making it not error for some machine types.   This isn't
> pretty, but if it's important for you to get the coniguration working
> then it's the type of trick that might work.
> 
> Dave
> 
> > 
> > > Fixes: 61964c23e5ddd5a33f15699e45ce126f879e3e33
> > > Cc: address@hidden
> > > Signed-off-by: Greg Kurz <address@hidden>
> > > ---
> > >  hw/ppc/spapr.c |    1 +
> > >  1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > index 5bd8fd3ef842..bca7cb8a5d27 100644
> > > --- a/hw/ppc/spapr.c
> > > +++ b/hw/ppc/spapr.c
> > > @@ -2446,6 +2446,7 @@ static void 
> > > spapr_machine_2_3_instance_options(MachineState *machine)
> > >      spapr_machine_2_4_instance_options(machine);
> > >      savevm_skip_section_footers();
> > >      global_state_set_optional();
> > > +    savevm_skip_configuration();
> > >  }
> > > 
> > >  static void spapr_machine_2_3_class_options(MachineClass *mc)
> > > 
> > > 
> > 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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