Re: [Qemu-devel] [RFC PATCH 1/3] target-i386: Add 486sx, old486, and old486sx CPU models

[H. Peter Anvin]

Qemu is absolutely horrid at modeling corner cases.

Rob Landley <address@hidden> wrote:

>On 03/28/2013 03:12:11 PM, H. Peter Anvin wrote:
>> On 03/28/2013 12:15 PM, Aurelien Jarno wrote:
>> >
>> > This really looks like Linux kernel specific. I haven't been able
>to
>> > test on a real machine, but the documentation I have found suggest 
>
>> that
>> > without and x87 FPU, the FPU instructions are simply ignored. The  
>> common
>> > way to detect an FPU is therefore to initialize registers to a
>given
>> > value, run fnstsw and fnstcw instructions with the register in  
>> arguments
>> > and see if they have been modified.
>> >
>> > The Linux kernel indeed set the initial value of these registers to
>> > 0xffff, but I am not sure all codes are doing the same.
>> >
>> > For me it looks like better to skip such instructions directly in
>> > translate.c. As a bonus it seems easy to do that for all FPU
>> > instructions.
>> >
>> 
>> It might have been (and this is from memory, so don't take it for
>> anything) that the register form receives 0xffff, but the memory form
> 
>> is
>> ignored.
>
>Speaking of which, Solar Designer recently found a bug where pentium 3 
>
>silently ignores the 66 prefix that later became SSE2, and thus the  
>code ran but produced the wrong result:
>
>https://twitter.com/solardiz/status/316204216962142209
>https://twitter.com/solardiz/status/316207184134410240
>
>But this isn't what QEMU does:
>
>https://twitter.com/solardiz/status/316944417871245313
>
>Rob

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