Re: [Qemu-ppc] [Qemu-devel] [PATCH 2/4] ppc: add CPU IRQ state to PPC VMStateDescription

[Mark Cave-Ayland]

On 13/09/17 07:02, David Gibson wrote:

>>> Alexey - do you recall from your analysis why these fields were no
>>> longer deemed necessary, and how your TCG tests were configured?
>>
>> I most certainly did not do analysis (my bad. sorry) - I took the patch
>> from David as he left the team, fixed to compile and pushed away. I am also
>> very suspicions we did not try migrating TCG or anything but pseries. My
>> guest that things did not break (if they did not which I am not sure about,
>> for the TCG case) because the interrupt controller (XICS) or the
>> pseries-guest took care of resending an interrupt which does not seem to be
>> the case for mac99.
> 
> Right, that's probably true.  The main point, though, is that these
> fields were dropped a *long* time ago, when migration was barely
> working to begin with.  In particular I'm pretty sure most of the
> non-pseries platforms were already pretty broken for migration
> (amongst other things).
> 
> Polishing the mac platforms up to working again, including migration,
> is a reasonable goal.  But it can't be at the expense of pseries,
> which is already working, used in production, and much better tested
> than mac99 or g3beige ever were.

Oh I completely agree since I'm well aware pseries likely has more users
than the Mac machines - my question was directed more about why we
support backwards migration.

I spent several hours yesterday poking my Darwin test case with trying
the different combinations of pending_interrupts, irq_input_state and
access_type and could easily provoke migration failures unless all 3 of
the fields were present so a practical test shows they are still
required for TCG migration. I think ppc_set_irq()'s use of the interrupt
fields in hw/ppc/ppc.c and the subsequent reference to pending
interrupts in target/ppc may explain why I see freezes/hangs until a key
is pressed in many cases.


ATB,

Mark.