Re: [Qemu-devel] [RFC PATCH v0 2/5] cpu: Optionally use arch_id instead of cpu_index in cpu vmstate_register()

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Wed, Jul 06, 2016 at 07:25:44AM +0200, Igor Mammedov wrote:
> On Tue, 5 Jul 2016 18:13:22 +0530
> Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> 
> > On Tue, Jul 05, 2016 at 09:15:51AM +0200, Igor Mammedov wrote:
> > > On Tue,  5 Jul 2016 10:12:49 +0530
> > > Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> > > 
> > > > Introduce CPUState.prefer_arch_id_over_cpu_index and
> > > > MachineClass.prefer_arch_id_over_cpu_index that allow target
> > > > machines to optionally switch to using arch_id instead of
> > > > cpu_index as instance_id in vmstate_register(). This will help
> > > > allow successful migration in cases where holes are introduced in
> > > > cpu_index range after CPU hot removals.
> > > > 
> > > > Whether to use arch_id or cpu_index is based on machine type
> > > > version and hence added
> > > > MachineClass.prefer_arch_id_over_cpu_index. However the
> > > > enforcement is via and during CPU creation and hence added
> > > > CPUState.prefer_arch_id_over_cpu_index. So it becomes a two step
> > > > process for the target to enable the use of arch_id:
> > > > 
> > > > 1. Set MachineClass.prefer_arch_id_over_cpu_index.
> > > > 2. Ensure CPUState.prefer_arch_id_over_cpu_index is set for all
> > > > CPUs based on 1. above.
> > > > 
> > > > Suggested-by: Igor Mammedov <imammedo@redhat.com>
> > > > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > > > ---
> > > >  exec.c              | 10 ++++++++--
> > > >  include/hw/boards.h |  1 +
> > > >  include/qom/cpu.h   |  4 ++++
> > > >  3 files changed, 13 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/exec.c b/exec.c
> > > > index 8ce8e90..7cc1d06 100644
> > > > --- a/exec.c
> > > > +++ b/exec.c
> > > > @@ -616,15 +616,21 @@ static void cpu_release_index(CPUState *cpu)
> > > >      bitmap_clear(cpu_index_map, cpu->cpu_index, 1);
> > > >  }
> > > >  
> > > > +/*
> > > > + * TODO: cpu_index and instance_id are of type int
> > > > while .get_arch_id()is
> > > > + * of type int64_t. What is the consequence of changing
> > > > instance_id to int64_t ?
> > > > + */
> > > ARM potentially could have get_arch_id() == MPIDR (return 64bit int)
> > > that would get truncated in this case.
> > > 
> > > I wonder if we could add "int CPUState::migration_id" and let
> > > machine code set it to what it uses for stable cpu id and then
> > > cpu_realize could call vmstate_register with it.
> > 
> > Hmm how would that help ? instance_id argument to vmstate_register()
> > is being treated as 32 bit integer as pointed by Greg earlier.
> > 
> > If we want to use 64 bit arch_id/migration_id as instance_id arg in
> > vmstate_register, shouldn't we be changing instance_id to 64 bit ?
> Trying to extend instance_id from 32bit to 64bit, looks hard to me as
> it's on wire format so it's not possible to change without breaking
> compatibility.
> 
> That's why dedicated migration_id should also be 32bit integer to keep
> it compatible with vmstate_foo() magic. On x86 and ARM that id will be
> in range [0..maxcpus), it's stable index in possible_cpus array.

Since we were planning to use arch_id (aka device tree id) as the stable
index for CPU cores on PowerPC, we would have to use int64_t arch_id
itself as int instance_id I suppose. The range could be
[0...maxcpus*8] for us.

Regards,
Bharata.