Re: [Xen-devel] [PATCH][SVM] remove FFXSR CPUID bit for AMD-V HVM guests

["Christoph Egger" <Christoph.Egger@xxxxxxx>]

On Thursday 01 February 2007 17:12, Jan Beulich wrote:
> >There is no known issue with FFSRX at this time, an alternative (that
> >works) is to modify the code in long_mode_do_msr_write() to not gp fault
> >on FFSRX bit being set (this was the original failure).
> >
> >        if ( msr_content & ~(EFER_FFSRX | EFER_LME | EFER_LMA | EFER_NX
> >
> >| EFER_SCE) )
> >
> >        {
> >            gdprintk(XENLOG_WARNING, "Trying to set reserved bit in "
> >                     "EFER: %"PRIx64"\n", msr_content);
> >            goto gp_fault;
> >        }
> >
> >The above code also allows windows to continue installation and function
> >seemingly properly.
>
> Then I'd favor this change over the posted one.
>
> >So, to answer your Q, there is not a particular known failure case for
> >3DNow! Or FFSRX disablement - the issue would be that there has been no
> >directed testing effort concerning 3DNow and FFSRX usages in the guest
> >to determine if would be any issues per se.
>
> For FFSRX, I can't see what issues you would expect. For 3dnow, it's as
> good (or as bad) as other MMX or XMM stuff trying to access MMIO, I would
> guess: if any of this is used anywhere, I guess some updates to emulation
> might be needed.

mplayer uses SIMD instructions pretty heavy for video decoding.
But I can't say if this leads to MMIO accesses w/o investigation.

Christoph




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