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

[Greg Kurz <groug@kaod.org>]

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On Wed, 13 Sep 2017 17:44:54 +0100
Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> wrote:

> 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.
> 

Downstream support backward migration because end users/customers ask for it
for maximum flexibility when it comes to move workloads around different systems
with different QEMU versions. This is fairly usual in data centers with many
systems.

As others already said, breaking things upstream may turn downstream work
into a nightmare (and FWIW, most of the people working on ppc are also
involved in downstream work).

Cheers,

--
Greg

> 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.


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