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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Intel SDM Volume 1 Section 8.6 (³X87 FPU Exception Synchronization²) is very
helpful in this regard. It confirms my suspicions, that we will need to wrap
the FPU inline asms in some fixup glue. The only exceptions to this rule are
the FN* (no-wait) instructions. Properly speaking we should also check for
unmasked exceptions becoming pending after any FPU instruction and set the
faulting CS:IP in the FPU context block appropriately. But life is perhaps
too short. :-)

 -- Keir

On 1/3/08 08:46, "Keir Fraser" <keir.fraser@eu.citrix.com> wrote:

> 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?



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