Re: [Qemu-devel] [RFC PATCH] target-ppc: enable migration within the same CPU family

[Michael Mueller <mimu@linux.vnet.ibm.com>]

On Tue, 08 Apr 2014 11:23:14 +1000
Alexey Kardashevskiy <aik@ozlabs.ru> wrote:

> On 04/08/2014 04:53 AM, Andreas Färber wrote:
> > Am 07.04.2014 05:27, schrieb Alexey Kardashevskiy:
> >> On 04/04/2014 11:28 PM, Alexander Graf wrote:
> >>> On 04/04/2014 07:17 AM, Alexey Kardashevskiy wrote:
> >>>> On 03/24/2014 04:28 PM, Alexey Kardashevskiy wrote:
> >>>>> Currently only migration fails if CPU version is different even a bit.
> >>>>> For example, migration from POWER7 v2.0 to POWER7 v2.1 fails because of
> >>>>> that. Since there is no difference between CPU versions which could
> >>>>> affect migration stream, we can safely enable it.
> >>>>>
> >>>>> This adds a helper to find the closest POWERPC family class (i.e. first
> >>>>> abstract class in hierarchy).
> >>>>>
> >>>>> This replaces VMSTATE_UINTTL_EQUAL statement with a custom handler which
> >>>>> checks if the source and destination CPUs belong to the same family and
> >>>>> fails if they are not.
> >>>>>
> >>>>> This adds a PVR reset to the default value as it will be overwritten
> >>>>> by VMSTATE_UINTTL_ARRAY(env.spr, PowerPCCPU, 1024).
> >>>>>
> >>>>> Since the actual migration format is not changed by this patch,
> >>>>> @version_id of vmstate_ppc_cpu does not have to be changed either.
> >>>>>
> >>>>> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> >>>>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> >>>>
> >>>> Ping?
> >>>
> >>> Can't we just always allow migration to succeed? It's a problem of the tool
> >>> stack above if it allows migration to an incompatible host, no?
> >>
> >> This is not how libvirt works. It simply sends the source XML, reconstructs
> >> a guest on the destination side and then migrates. hoping that the
> >> migration will fail is something (which only QEMU has knowledge of) is
> >> incompatible. The new guest will start with "-cpu host" (as the source) but
> >> it will create diffrent CPU class and do different things. If we do not
> >> check PVR (and cpu_dt_id and chip_id - the latter is coming soon) and
> >> migrate power8->power7, we can easily get a broken guest.
> > 
> > The response is very simple: -cpu host is not supported for migration.
> > Same as for x86 hosts.
> 
> Is there any good reason to limit ourselves on POWERPC?
> 
> > As you say, the domain config is transferred by libvirt:
> > If you use -cpu POWER7, you can migrate from POWER7 to POWER8 and back;
> > if you use -cpu POWER8, you can only migrate on POWER8.
> 
> -cpu other that "host" is not supported by HV KVM, only "compat" which
> upstream QEMU does not have yet. So you are saying that the migration is
> not supported by upstream QEMU for at least SPAPR. Well, ok, it is dead
> anyway so I am fine :)
> 

With s390x we have a similar situation. Thus we came up with a mechanism to limit
the CPU functionality of a possible target system. Our patch implements CPU models
based on TYPE and GA like 2817-ga1, etc. (GA represents a CPU facility set and an IBC
value (Instruction Blocking Control, reduces the instruction set to the requested
level)) When a guest is started, it receives its CPU model by means of option -cpu.
"host" equates the configuration of the current system. We implemented "query-cpu-model"
returning the actual model, here maybe { name: "2817-ga1" }. To find a suitable
migration target in a remote CEC, libvirt has to "query-cpu-definitions" returning a
list of models supported by the target system "{{name: "2827-ga2"}, {name: "2827-ga1"},
{name: "2817-ga2"},...]. A match means the system is suitable and can be used
as migration target.

Thanks
Michael