Re: [Qemu-devel] [PATCH 4/4] target-ppc: ensure we include the decrementer value during migration

[Mark Cave-Ayland]

On 12/01/16 02:44, David Gibson wrote:

>>> In other words, isn't this just skipping the decrementer interrupts at
>>> the qemu level rather than the guest level?
>>>
>>> It seems that instead we should be reconstructing the decrementer on
>>> the destination based on an offset from the timebase.
>>
>> Well I haven't really looked at how time warping works during in
>> migration for QEMU, however this seems to be the method used by
>> hw/ppc/ppc.c's timebase_post_load() function but my understanding is
>> that this isn't currently available for the g3beige/mac99 machines?
> 
> Ah.. yes, it looks like the timebase migration stuff is only hooked in
> on the pseries machine type.  As far as I can tell it should be
> trivial to add it to other machines though - it doesn't appear to rely
> on anything outside the common ppc timebase stuff.
> 
>> Should the patch in fact do this but also add decrementer support? And
>> if it did, would this have a negative effect on pseries?
> 
> Yes, I think that's the right approach.  Note that rather than
> duplicating the logic to adjust the decrementer over migration, it
> should be possible to encode the decrementer as a diff from the
> timebase across the migration.
> 
> In fact.. I'm not sure it ever makes sense to store the decrementer
> value as a direct value, since it's constantly changing - probably
> makes more sense to derive it from the timebase whenever it is needed.
> 
> As far as I know that should be fine for pseries.  I think the current
> behaviour is probably technically wrong for pseries as well, but the
> timing code of our Linux guests is robust enough to handle a small
> displacement to the time of the next decrementer interrupt.

I've had a bit of an experiment trying to implement something suitable,
but I'm not 100% certain I've got this right.

>From the code my understanding is that the timebase is effectively
free-running and so if a migration takes 5s then you use tb_offset to
calculate the difference between the timebase before migration, and
subsequently apply the offset for all future reads of the timebase for
the lifetime of the CPU (i.e. the migrated guest is effectively living
at a point in the past where the timebase is consistent).

So I do see that it would make more sense to switch the decrementer over
to an offset from timebase to avoid duplicating the time-shift migration
logic, but my PPC experience is fairly limited. Any particular pointers
as to what is the best way to approach this? It seems getting this done
separately first will make the changes in the last patch trivial.


Many thanks,

Mark.