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Re: [Xen-devel] fma4/avx under xen

Jan Beulich <JBeulich@xxxxxxxx> wrote [05.04.12]:
> >>> On 02.05.12 at 20:54, Jim Westfall <jwestfall@xxxxxxxxxxxxxxxx> wrote:
> > The summary is that when gcc compiles something with -mfma4 it also 
> > enables the use of the avx instruction set.  Since by default xen 
> > disables avx it leads to invalid opcodes.
> Not that I know of, at least not anymore in current -unstable.
> > This ends up being kinda nasty with the multiarch glibc since its doing 
> > stuff like
> > 
> > (HAS_FMA4 ? run_fma4_func() : (HAS_AVX ? run_avx_func() : run_func()))
> The question is what (runtime) tests HAS_FMA includes. I know that
> some glibc versions had a broken AVX check (which only checked for
> the AVX feature flag, while the specification explicitly states that
> a prerequisite check of the OSXSAVE feature flag is also necessary).

Its just a basic cpuid() call and seeing if the FMA4 bit is set.  I will 
bug the glibc guys.  Thanks for the feedback.


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