Re: [PATCH 3/4] x86: don't use FPU instructions when there is no FPU

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 14/10/15 14:31, Jan Beulich wrote:
> --- a/xen/include/asm-x86/cpufeature.h
> +++ b/xen/include/asm-x86/cpufeature.h
> @@ -170,6 +170,7 @@
>  #define CPUID5_ECX_EXTENSIONS_SUPPORTED 0x1
>  #define CPUID5_ECX_INTERRUPT_BREAK      0x2
>  
> +#define cpu_has_fpu		boot_cpu_has(X86_FEATURE_FPU)

This distinguishes only between an onboard fpu or an original x87
co-processor.  It does not represent the presence or absence of an fpu.

Looking at the Xeon Phi documentation
(http://download-software.intel.com/sites/default/files/forum/278102/327364001en.pdf),
it has this feature bit set, although it does have MMX clear.

Interestingly however, a Phi's default MXCSR value ix 0x20000h and will
#GP at any attempt to clear bit 21, which means fpu_init() will
unconditionally blow up on such a system.

I am not aware of a bit which signifies the absence of FPU
capabilities.  Until there is, I don't think changes along these lines
are a safe way to go.

~Andrew