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Re: [Qemu-ppc] [Qemu-devel] [PATCH] PPC: Depend behavior of cmp instruct

From: Aurelien Jarno
Subject: Re: [Qemu-ppc] [Qemu-devel] [PATCH] PPC: Depend behavior of cmp instructions only on instruction encoding
Date: Wed, 8 May 2013 17:04:12 +0200
User-agent: Mutt/1.5.20 (2009-06-14)

On Wed, May 08, 2013 at 04:48:22PM +0200, Torbjorn Granlund wrote:
> Aurelien Jarno <address@hidden> writes:
>   That said this does implement neither the specification nor the silicon
>   behaviour. This is fine for 1.5 as we are in freeze period, but this 
>   should be fixed for the 1.6 release.
> I talked to IBM now.  Reserved fields should be ignored by hardware.

As it seems you have good contact with IBM, could you please ask them
to fix their manuals?

> The architecture owner is IBM, not Freescale.  That Freescale deviates
> from the architecture, is something that you may decide to ignore,
> unless it is vital for qemu's behaviour in practice.

At least Freescale CPUs matches what IBM documentation says. IBM CPUs

> I very much doubt that L = 1 often, for code targeting a 32-bit
> processor.
> Trying to mimic decoding flaws on a per-processor basis, is going to
> take a lot of research, and will be prone to errors.
> So as far as I can tell, the patch is correct as per the architecture
> specification.

No it's not correct, it doesn't match neither Freescale nor IBM
behaviour. It also means the same code executed on a 32-bit emulated CPU
run with qemu-system-ppc will behave differently than when run with
qemu-system-ppc64. This is fine for now as we are in freeze period, but
should be fixed afterwards.

> One caveat though: Does 32-bit implementations define the SF bit, or
> else, does qemu define it and make sure it is 0 for 32-bit emulation?
> If not, the patch might cause trouble.

QEMU makes sure it is 0 for 32-bit CPU.

> Congrats, you read a "user message" until the last line.  :-)

Like I did for the previous one. Would be nice if you can do the same.

Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net

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