[Xen-devel] Re: [PATCH] [RFC] More fp instructions for realmode emulation (Enables booting OS/2 as a HVM guest on Intel/VT hardware)

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 3/3/08 16:59, "Andi Kleen" <andi@xxxxxxxxxxxxxx> wrote:

>>> Yes you definitely need to. That was a kernel crash bug on native Linux
>>> some time ago...
>>> 
>>> Also you need to do the same for any FXSAVEs/FXRSTORs if you haven't
>>> already.
>> 
>> FXSAVE/FXRSTOR do not flush pending FPU exceptions, right?
> 
> Yes they do not, but there are other ways to make them #GP

We don't trust FXRSTOR but we do trust FXSAVE. The AMD manual says that
FXRSTOR will #GP if 1s are written to MXCSR reserved bits, but I take this
to be a cut-and-paste typo since FXSAVE does not write to MXCSR.

If there's real evidence of FXSAVE causing #GP dependent on current FPU
state, I'd be very happy to find out about it!

 -- Keir



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