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Date: Wed, 02 Oct 2013 07:22:09 +0200
From: Stefan Weil <sw@weilnetz.de>
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Subject: Re: [Qemu-devel] [Bug 1233225] Re: mips/mipsel linux user float
 division problem
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To: Peter Maydell <peter.maydell@linaro.org>, Bug 1233225 <1233225@bugs.launchpad.net>
Cc: QEMU Developers <qemu-devel@nongnu.org>, =?UTF-8?B?QXVyw6lsaWVuIEphcm4=?= =?UTF-8?B?bw==?= <aurelien@aurel32.net>, Richard Henderson <rth@twiddle.net>

Am 02.10.2013 03:39, schrieb Peter Maydell:
> On 2 October 2013 02:51, Stefan Weil <1233225@bugs.launchpad.net> wrote:
>> I can confirm that something is strange with MIPS Linux user emulation,
>> but get a different result (which is also wrong):
>>
>> # Your test code is in file divtest.c.
>> $ mipsel-linux-gnu-gcc-4.7 -g -static divtest.c
>> $ mipsel-linux-user/qemu-mipsel a.out
>> 0.000000
> Does the CPU you're asking qemu to emulate match the CPU gcc is
> generating code for? IIRC for MIPS there's no single "right" answer
> for "which CPU do we default to"...
>
> -- PMM
>

QEMU user mode emulation for MIPS knows two different default cpus:

#if defined(TARGET_ABI_MIPSN32) || defined(TARGET_ABI_MIPSN64)
        cpu_model = "20Kc";
#else
        cpu_model = "24Kf";
#endif

In this case, it took 24Kf which seems to be ok.

I tested other cpus with -cpu xxx. They either show the same result
or fail with illegal instruction.

This is my executable:

divtest-mips: ELF 32-bit MSB executable, MIPS, MIPS-II version 1,
statically linked, for GNU/Linux 2.6.26

The original bug report said that it runs in QEMU system emulation
(which I did not test because of lack of time). As system emulation
uses the same cpu, it should be fine.

See also my previous mail: bit CP0St_FR makes a difference:
http://lists.nongnu.org/archive/html/qemu-devel/2013-10/msg00168.html

Stefan