Re: [Qemu-devel] [PATCH 3/3] ppc: include timebase in migration stream for g3beige/mac99 machines

[David Gibson <david@gibson.dropbear.id.au>]

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On Sun, Jan 31, 2016 at 08:10:08PM +0000, Mark Cave-Ayland wrote:
> On 31/01/16 19:58, Peter Maydell wrote:
> 
> > On 31 January 2016 at 19:19, Mark Cave-Ayland
> > <mark.cave-ayland@ilande.co.uk> wrote:
> >> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> >> ---
> >>  hw/ppc/mac_newworld.c |    4 ++++
> >>  hw/ppc/mac_oldworld.c |    4 ++++
> >>  2 files changed, 8 insertions(+)
> >>
> >> diff --git a/hw/ppc/mac_newworld.c b/hw/ppc/mac_newworld.c
> >> index f95086b..3283f1d 100644
> >> --- a/hw/ppc/mac_newworld.c
> >> +++ b/hw/ppc/mac_newworld.c
> >> @@ -179,6 +179,7 @@ static void ppc_core99_init(MachineState *machine)
> >>      int *token = g_new(int, 1);
> >>      hwaddr nvram_addr = 0xFFF04000;
> >>      uint64_t tbfreq;
> >> +    PPCTimebase *tb;
> >>
> >>      linux_boot = (kernel_filename != NULL);
> >>
> >> @@ -201,6 +202,9 @@ static void ppc_core99_init(MachineState *machine)
> >>          /* Set time-base frequency to 100 Mhz */
> >>          cpu_ppc_tb_init(env, TBFREQ);
> >>          qemu_register_reset(ppc_core99_reset, cpu);
> >> +
> >> +        tb = g_malloc0(sizeof(PPCTimebase));
> >> +        vmstate_register(NULL, -1, &vmstate_ppc_timebase, tb);
> > 
> > Is there no way to avoid the vmstate_register here (ie to
> > tie the migration data to an actual device or CPU object) ?
> 
> Not exactly that I know of - although I shamelessly borrowed this part
> from similar code in spapr which has this comment:
> 
>     /* FIXME: Should register things through the MachineState's qdev
>      * interface, this is a legacy from the sPAPREnvironment structure
>      * which predated MachineState but had a similar function */
> 
> Is this something that is now possible?

Well, it's certainly possible to do better than this.  You want to
make a vmstate_g3beige and vmstate_mac99 which contain all the machine
level things to migrate for these machines, similar to vmstate_spapr.
They will be attached to the MachineState object.

That will at least mean that if more things need to get added to
migration for these machines, then additional vmstate_register() calls
won't be needed.

I'm not sure if there's a better way to register a vmstate for a
machine type.  I thought there was, but I couldn't spot it in a quick
lock.


Peter,

I believe this does need to be attached to the machine, not to the
cpu, even though the cpu would seem to make more sense on a first
look.  The reason is that attaching it to the cpu means it will be
transferred separately for each cpu, and unless we're super-careful
about timing the destination cpus could end up with slightly different
values.  That would be bad, because ppc has a pretty strong
requirement that the timebases be synchronized across all cpus in an
smp system.  The means of initially accomplishing that vary by
platform - usually there's some board level register to freeze /
resume all the timebases - but however it's been done, we don't want
to mess it up on migration.

-- 
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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