Re: [Qemu-devel] [PATCH 2/2] pseries: Use new hook to correct reset sequence

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

On Wed, Aug 08, 2012 at 05:22:11PM +0200, Andreas Färber wrote:
> Am 08.08.2012 03:45, schrieb David Gibson:
> > On Wed, Aug 08, 2012 at 12:32:39AM +0200, Andreas Färber wrote:
> >> Am 08.08.2012 00:02, schrieb Benjamin Herrenschmidt:
> >>> On Fri, 2012-08-03 at 17:01 +0200, Andreas Färber wrote:
> >>>>
> >>>> I have posted a suggestion where CPU reset is triggered by "the
> >>>> machine
> >>>> as an abstract concept" (needs a bit of tweaking still, but the
> >>>> general
> >>>> idea is there).
> >>>> Based on that, shouldn't it be rather easy to add a Notifier similar
> >>>> to
> >>>> "machine init done" that lets individual machines do post-reset setup?
> >>>> I.e. not have QEMUMachine trigger and control the reset.
> >>>>
> >>>
> >>> Note that we really want pre and post reset vs the device reset.
> >>>
> >>> That's why the machine should be the one in charge. The top level of the
> >>> reset sequencing is -not- the CPU, it's the machine. All machines (or
> >>> SoCs) have some kind of reset controller and provide facilities for
> >>> resetting individual devices, busses, processor cores.... the global
> >>> "system" reset (when it exists) itself might have interesting ordering
> >>> or sequencing requirements.
> >>>
> >>> Now, to fix our immediate problem on ppc for 1.2 the hook proposed by
> >>> Anthony for which David sent a patch does the job just fine, it allows
> >>> us to clean out all our iommu tables before the device-reset, meaning
> >>> that in-flights DMA cannot overwrite the various "files" (SLOF image
> >>> etc.... that are auto-loaded via reset handlers implicitely created by
> >>> load_image_targphys), and we can then do some post-initializations as
> >>> well to get things ready for a restart (rebuild the device-tree, etc...)
> >>
> >> That's all good, except for embedded machines without such implicit
> >> reset handling. It does contradict the "a machine is just a config file,
> >> setting up QOM objects" concept, but I was not the one to push that! :)
> >>
> >> What I was thinking about however were those mentioned individual cores
> >> being reset using cpu_reset(). If we want to piggy-back some
> >> machine-specific register initialization for individual CPUStates then
> >> QEMUMachine::reset is not going to be enough because it only gets
> >> triggered for complete system reset. My suggestion was thus to just call
> >> cpu_reset() in your QEMUMachine::reset and have cpu_reset() take care of
> >> its initialization wherever called from. Any of these solutions are easy
> >> to implement for 1.2 if agreement is reached what people want.
> > 
> > So, I more or less reaslied that myself and my new version of the
> > reset patch (which I expect to send out later today) kind of does
> > that.  I no longer do the machine specific CPU state setup from the
> > QEMUMachine::reset, it's done from the per-cpu reset handler.  The
> > QEMUMachine::reset just does the special setup that's only for the
> > CPU0 entry conditions, which *is* specific to a full system reset (not
> > that I think we can get an individual CPU reset on pseries, anyway).
> > 
> >> What I am missing from Anthony's side is some communication to machine
> >> maintainers on the course to adopt before applying random patches. Right
> >> now x86 and ppc are moving into opposite directions and arm, mips, etc.
> >> maintainers may not even be aware of ongoing changes, and there's a
> >> pending uc32 machine that should be reviewed in this light.
> > 
> > So.. having the CPU reset at the top of the tree definitely makes no
> > sense - if nothing else, *which* cpu when there's more than one.
> 
> Maybe let me restate clearly what I am looking for in this discussion:
> 
> I would like a clear definition of
> * what is the "normal" case, and
> * what is the special case.
> 
> The special case sPAPR seems uncontroversial.
> 
> So, a bonus would be if we can have a default implementation (of
> QEMUMachine::reset or whatever we end up doing) so that the average
> machine does not need to fiddle with reset callbacks in
> QEMUMachine::init. For example, have a machine_default_reset() as
> fallback for QEMUMachine::reset == NULL that resets all CPUs (in order
> of the singly linked list) and then does qemu_devices_reset()? sPAPR
> would then override that default implementation by specifying its own
> implementation and we could get rid of reset callbacks in an estimated
> 70% of QEMUMachine::init. (The less people fiddle at that level the
> easier to refactor for me.) That could well be a later follow-up to your
> v2, which looked okay on brief sight.

We already have that.  If QEMUMachine::reset is NULL,
qemu_system_reset() does qemu_devices_reset() which is exactly the
same as what it did before.  qemu_devices_reset() calls all the reset
callback handlers, so it will also reset the CPUs if a suitable CPU
reset handler has been registered.

-- 
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