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Date: Thu, 4 Apr 2013 13:58:14 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 07/11] versatile_pci: Implement the
 correct PCI IRQ mapping
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>, patches@linaro.org, Will Deacon <will.deacon@arm.com>, qemu-devel@nongnu.org, Paul Brook <paul@codesourcery.com>, Andreas =?iso-8859-1?Q?F=E4rber?= <afaerber@suse.de>, Aurelien Jarno <aurelien@aurel32.net>

On Thu, Apr 04, 2013 at 12:05:51PM +0100, Peter Maydell wrote:
> On 28 March 2013 10:28, Peter Maydell <peter.maydell@linaro.org> wrote:
> > On 26 March 2013 21:12, Michael S. Tsirkin <mst@redhat.com> wrote:
> >> On Tue, Mar 26, 2013 at 11:17:55AM +0000, Peter Maydell wrote:
> >>> I'm happy to provide some other way for QEMU to detect a
> >>> new working kernel if you want to implement one in the
> >>> kernel, if that's cleaner.
> >>
> >> That's why I suggested writing some number at offset 0x08
> >> (that's revision ID) in configuration space of device 0 on bus 0.
> >> It's guaranteed harmless on real hardware and QEMU could detect this
> >> write and say "aha new kernel, even though it uses #27".
> >
> > OK, that makes sense.
> 
> In fact I've just realised that "allow new kernels to use #27
> without getting the back-compat broken irq mapping" conflicts
> with "allow new kernels to kexec old broken kernels".

Could you clarify why, pls?  Also - does this really matter so much?
kexec between version is always a risky affair...

> So the
> easiest approach is just to drop the code which allows us to
> switch back to broken mode again. Then the new kernel's init
> code can force non-broken mode by writing something other than
> #27 to some device's PCI_INTERRUPT_LINE register, and then
> is free to use #27 if it wants to.
> 
> -- PMM

I'm fine with this too.