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

[Rob Landley <rob@landley.net>]

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