Re: [PATCH v4 3/3] x86: short-circuit certain cpu_has_* when x86-64-v{2,3} are in effect

[Roger Pau Monné <roger@xxxxxxxxxxxxxx>]

On Thu, Oct 15, 2026 at 11:11:39AM +0200, Jan Beulich wrote:
> Certain fallback code can be made subject to DCE this way. Note that
> CX16 has no compiler provided manifest constant, so CONFIG_* are used
> there instead. Note also that we don't have cpu_has_movbe nor
> cpu_has_lzcnt (aka cpu_has_abm), and we no longer have
> cpu_has_{popcnt,bmi1,bmi2}.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Jason Andryuk <jandryuk@xxxxxxxxx>
> ---
> Of course we could use IS_ENABLED(CONFIG_X86_64_V<n>) everywhere, but as
> CX16 shows this isn't necessarily better than the #if/#else approach
> based on compiler-provided manifest symbols. While not really intended
> to be used that way, it looks as if we could also use
> IS_ENABLED(__POPCNT__) and alike if we thought this would end up neater
> (because of avoiding the #ifdef).
> 
> We could go further and also short-circuit SSE*, AVX and alike, which we
> don't use outside of the emulator. This then would of course call for
> also having a way to select x86-64-v4.
> ---
> v4: Re-base (over dropped cpu_has_{popcnt,bmi1,bmi2}).
> v2: Also cover XSAVE.
> 
> --- a/xen/arch/x86/include/asm/cpufeature.h
> +++ b/xen/arch/x86/include/asm/cpufeature.h
> @@ -132,11 +132,17 @@ static inline bool boot_cpu_has(unsigned
>  #define cpu_has_monitor         boot_cpu_has(X86_FEATURE_MONITOR)
>  #define cpu_has_vmx             boot_cpu_has(X86_FEATURE_VMX)
>  #define cpu_has_eist            boot_cpu_has(X86_FEATURE_EIST)
> -#define cpu_has_cx16            boot_cpu_has(X86_FEATURE_CX16)
> +#define cpu_has_cx16            (IS_ENABLED(CONFIG_X86_64_V2) || \
> +                                 IS_ENABLED(CONFIG_X86_64_V3) || \
> +                                 boot_cpu_has(X86_FEATURE_CX16))
>  #define cpu_has_pdcm            boot_cpu_has(X86_FEATURE_PDCM)
>  #define cpu_has_pcid            boot_cpu_has(X86_FEATURE_PCID)
>  #define cpu_has_x2apic          boot_cpu_has(X86_FEATURE_X2APIC)
> +#ifdef __XSAVE__
> +#define cpu_has_xsave           true
> +#else
>  #define cpu_has_xsave           boot_cpu_has(X86_FEATURE_XSAVE)
> +#endif
>  #define cpu_has_avx             boot_cpu_has(X86_FEATURE_AVX)
>  #define cpu_has_rdrand          boot_cpu_has(X86_FEATURE_RDRAND)
>  #define cpu_has_hypervisor      boot_cpu_has(X86_FEATURE_HYPERVISOR)

Alejandro (now added on Cc) is working on something that achieves the
same end, but it's more transparent and doesn't require modifying
individual cpu_has_ macros every time we want to short-circuit one of
those.  I think it would be helpful here also.

Regards, Roger.