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From: Anthony Liguori <anthony@codemonkey.ws>
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Date: Fri, 03 Aug 2012 09:22:15 -0500
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Subject: Re: [Qemu-devel] [PATCH 2/2] pseries: Use new hook to correct reset
	sequence
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Igor Mammedov <imammedo@redhat.com>, agraf@suse.de, qemu-devel@nongnu.org, Andreas =?utf-8?Q?F=C3=A4r?= =?utf-8?Q?ber?= <afaerber@suse.de>, David Gibson <david@gibson.dropbear.id.au>

Peter Maydell <peter.maydell@linaro.org> writes:

> On 3 August 2012 14:50, Anthony Liguori <anthony@codemonkey.ws> wrote:
>> There ought to be a hierarchy (based on composition) that reset flows
>> through.
>
> I think saying "the reset tree is isomorphic to the composition tree"
> is making the same mistake that qbus did with "the bus tree is
> isomorphic to the composition tree". The stakes are lower for reset
> and we can probably get away with it, but it really isn't how the
> hardware works...

It flows through the composition tree by default, but can be overridden
at any point.

For instance, the i440fx will absolutely want to override this behavior
such that it can flow reset through the PCI bus (which is how the PIIX3
would be reset).  However, the PIIX3 has no need to override this
behavior.

So this model should work very well for most types of virtual hardware.
But it doesn't provide for a mechanism to "after all devices are
initialized, build FDT in guest memory, then set the CPU registers to
point to it".

There's no logical device that has a scope like that that also has the
mechanism to get that type of hook in the reset path.  That's why we
need to have the QEMUMachine::reset() hook.

Regards,

Anthony Liguori

>
> -- PMM