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@eu.citrix.com>]

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Regarding point 1: Don¹t FPU instructions that raise an exception only log
that in the status word, to be picked up by the next FPU instruction which
checks for exceptions? It strikes me that we can have previously loaded up a
status word that requires an exception to occur on FWAIT, in which case
semantics are broken. More worryingly, if any of the instructions you have
added can cause an exception, we will crash Xen because you have not added
fixup handling. Unfortunately I¹m not really up on exactly how FPU exception
handling works: are you?

Regarding point 2: Yes, direct use of the FPU instructions is fine as we
already loaded guest FPU context.

 -- Keir

On 29/2/08 22:14, "Trolle Selander" <trolle.selander@gmail.com> wrote:

> I'm a little concerned about the "correctness" of the FSTSW emulation and the
> use of inline assembly directly using the corresponding ops for emulation. Wrt
> FSTSW, it is really two ops FNSTSW immediately preceeded by an FWAIT. I've
> left FWAIT defined, but a no-op because after looking at realmode.c it seems
> in the emulation, exceptions are always processed if a call to x86_emulate
> returns an exception, so there should be no way for there to be "pending"
> exceptions for FWAIT to wait for. I may have missed something, though, hence
> the RFC.
> 
> Secondly, about the inline assembly - i saw that this was how many of the
> other FP instructions were done, so I did it that way as well. However, none
> of the previous instructions done that way referenced the fp registers. Does
> this make a difference, or is it ok the way i've done it here?



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