Re: [Qemu-devel] [PATCH v3 0/2] Fix migration of old pseries

[Greg Kurz]

On Tue, 23 Feb 2016 09:31:10 +0100
Juan Quintela <address@hidden> wrote:
> Greg Kurz <address@hidden> wrote:
> > On Fri, 19 Feb 2016 11:11:47 +1100
> > David Gibson <address@hidden> wrote:
> >  
> >> On Thu, Feb 18, 2016 at 12:32:11PM +0100, Greg Kurz wrote:  
> >> > QEMU 2.4 broke the migration of old pseries machine with the addition
> >> > of configuration sections, which are sent unconditionally.
> >> > 
> >> > We assume that QEMU 2.3 is more deployed than any newer release (based on
> >> > the versions currently shipped by most distros). This v3 series hence
> >> > reverses the logic from v2: it now fully fixes migration of old pseries
> >> > from/to QEMU 2.3 and provides a manual workaround for the QEMU 
> >> > 2.4/2.4.1/2.5
> >> > case.
> >> > 
> >> > With this series, I could migrate the same pseries-2.3 instance in a full
> >> > 2.3->2.6->2.5->2.6->2.4->2.6->2.3 cycle.    
> >> 
> >> Sorry, I've lost track slightly here.  Does this series apply on top
> >> of, or instead of your earlier series that peeks for the config
> >> section?
> >>   
> >
> > This v3 series applies instead of the v2 that peeks for the config section.
> >
> > It was suggested by Laurent during review, and motivated by your decision
> > to favor fixing 2.3 over 2.4.
> >
> > As shown in Laurent's detailed test report, migration from/to 2.3.x now 
> > works
> > out of the box and 2.4.x/2.5 requires qom-set.
> >
> > I was also feeling a bit uncomfortable with all these machine properties to
> > disable the configuration section, which was explicitly coded to be 
> > non-optional
> > according to the changelog of commit 61964c23. The logic inversion in v3 
> > seem
> > to be friendlier with the configuration section design.
> >
> > Juan, could you share your thoughts ?  
> 
> As said previously, we don't have any better fix.  We could try to make
> the "enforce" thing only for some machine types, but I guess that it
> would make the mess even bigger :-(
> 
> And on an unrelated topic, I guess we need to "agree" in a way to add
> new features to migration for all machine types.  Right now we have:
> - two x86 boards
> - now s390
> - and now ppc
> - and I guess arm is on the way
> 
> So, I guess we would need something like of what we have on
> hw/i386/pc_piix.c
> 
> 
> static void pc_compat_2_3(MachineState *machine)
> {
>     PCMachineState *pcms = PC_MACHINE(machine);
>     savevm_skip_section_footers();
>     if (kvm_enabled()) {
>         pcms->smm = ON_OFF_AUTO_OFF;
>     }
>     global_state_set_optional();
>     savevm_skip_configuration();
> }
> 
> 
> Really, we need something like this
> 
> static void migration_compat_2_3(MachineState *machine)
> {
>     savevm_skip_section_footers();
>     global_state_set_optional();
>     savevm_skip_configuration();
> }
> 

Indeed, there is code duplication here... and it is not the first time:
pseries is also missing a compat property for the vmdesc section. In fact,
people change migration and often fix x86 only in a followup patch.

> to be included in every machine that cares about migration.  Is there
> ane easy way/place where to do this globaly?
> 

There has been a first round of compat stuff refactoring with
commit 68a27b208a, which introduced include/hw/compat.h.

Maybe we can declare the migration_compat_* helpers in this header
and put the code in a new hw/core/compat.c file ? Or add this to
hw/core/machine.c ?

I'll give a try and send a series.

> Later, Juan.
> 

Thanks !

--
Greg