Re: [PATCH v3 2/6] x86: replace APERFMPERF synthetic feature bit

["Teddy Astie" <teddy.astie@xxxxxxxxxx>]

Le 14/01/2026 à 14:45, Jan Beulich a écrit :
> Use the respective host CPU policy bit instead. This has the (tolerable,
> as we generally assume symmetry anyway) effect of using the BSP's
> (unleveled) setting, rather than the result of leveling (as is done by
> identify_cpu() on boot_cpu_data, rendering the variable name somewhat
> misleading).
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> ---
> The leveling of boot_cpu_data is problematic anyway, as that way features
> can in principle disappear post-boot (as CPUs are being brought online;
> just that we don't think anymore that we really support physical CPU
> hotplug).

I think in the vast majority of cases, hotplugged CPUs are of same model
or very similar; so shouldn't diverge too much in term of features.
Otherwise, it's pretty much impossible to guarantee anything unless we
have per-socket cpu datas.

> ---
> v3: Re-base over naming changes.
> v2: Extend description.
>
> --- a/xen/arch/x86/acpi/cpufreq/cpufreq.c
> +++ b/xen/arch/x86/acpi/cpufreq/cpufreq.c
> @@ -80,7 +80,7 @@ unsigned int get_measured_perf(unsigned
>           return 0;
>
>       policy = per_cpu(cpufreq_cpu_policy, cpu);
> -    if ( !policy || !cpu_has_aperfmperf )
> +    if ( !policy || !cpu_has_hw_feedback_cap )
>           return 0;
>
>       switch (flag)
> --- a/xen/arch/x86/cpu/common.c
> +++ b/xen/arch/x86/cpu/common.c
> @@ -523,10 +523,6 @@ static void generic_identify(struct cpui
>       if ( cpu_has(c, X86_FEATURE_CLFLUSH) )
>               c->x86_clflush_size = ((ebx >> 8) & 0xff) * 8;
>
> -     if ( (c->cpuid_level >= CPUID_PM_LEAF) &&
> -          (cpuid_ecx(CPUID_PM_LEAF) & CPUID6_ECX_APERFMPERF_CAPABILITY) )
> -             __set_bit(X86_FEATURE_APERFMPERF, c->x86_capability);
> -
>       /* AMD-defined flags: level 0x80000001 */
>       if (c->extended_cpuid_level >= 0x80000001)
>               cpuid(0x80000001, &tmp, &tmp,
> --- a/xen/arch/x86/include/asm/cpufeature.h
> +++ b/xen/arch/x86/include/asm/cpufeature.h
> @@ -11,7 +11,9 @@
>   #include <xen/macros.h>
>
>   #ifndef __ASSEMBLER__
> +#include <asm/cpu-policy.h>
>   #include <asm/cpuid.h>
> +#include <xen/lib/x86/cpu-policy.h>
>   #else
>   #include <asm/cpufeatureset.h>
>   #endif
> @@ -121,7 +123,6 @@ static inline bool boot_cpu_has(unsigned
>   #define CPUID6_EAX_HDC                               BIT(13, U)
>   #define CPUID6_EAX_HWP_PECI                          BIT(16, U)
>   #define CPUID6_EAX_HW_FEEDBACK                       BIT(19, U)
> -#define CPUID6_ECX_APERFMPERF_CAPABILITY             BIT(0, U)
>
>   /* CPUID level 0x00000001.edx */
>   #define cpu_has_fpu             1
> @@ -175,6 +176,9 @@ static inline bool boot_cpu_has(unsigned
>   #define cpu_has_fma4            boot_cpu_has(X86_FEATURE_FMA4)
>   #define cpu_has_tbm             boot_cpu_has(X86_FEATURE_TBM)
>
> +/* CPUID level 0x00000006.ecx */
> +#define cpu_has_hw_feedback_cap host_cpu_policy.basic.hw_feedback_cap
> +
>   /* CPUID level 0x0000000D:1.eax */
>   #define cpu_has_xsaveopt        boot_cpu_has(X86_FEATURE_XSAVEOPT)
>   #define cpu_has_xsavec          boot_cpu_has(X86_FEATURE_XSAVEC)
> @@ -292,7 +296,6 @@ static inline bool boot_cpu_has(unsigned
>   /* Synthesized. */
>   #define cpu_has_arch_perfmon    boot_cpu_has(X86_FEATURE_ARCH_PERFMON)
>   #define cpu_has_cpuid_faulting  boot_cpu_has(X86_FEATURE_CPUID_FAULTING)
> -#define cpu_has_aperfmperf      boot_cpu_has(X86_FEATURE_APERFMPERF)
>   #define cpu_has_xen_lbr         boot_cpu_has(X86_FEATURE_XEN_LBR)
>   #define cpu_has_xen_shstk       (IS_ENABLED(CONFIG_XEN_SHSTK) && \
>                                    boot_cpu_has(X86_FEATURE_XEN_SHSTK))
> --- a/xen/arch/x86/include/asm/cpufeatures.h
> +++ b/xen/arch/x86/include/asm/cpufeatures.h
> @@ -19,7 +19,7 @@ XEN_CPUFEATURE(TSC_RELIABLE,      X86_SY
>   XEN_CPUFEATURE(XTOPOLOGY,         X86_SYNTH( 5)) /* cpu topology enum 
> extensions */
>   XEN_CPUFEATURE(CPUID_FAULTING,    X86_SYNTH( 6)) /* cpuid faulting */
>   XEN_CPUFEATURE(XEN_FRED,          X86_SYNTH( 7)) /* Xen uses FRED */
> -XEN_CPUFEATURE(APERFMPERF,        X86_SYNTH( 8)) /* APERFMPERF */
> +/* Bit 8 unused */
>   XEN_CPUFEATURE(MFENCE_RDTSC,      X86_SYNTH( 9)) /* MFENCE synchronizes 
> RDTSC */
>   XEN_CPUFEATURE(XEN_SMEP,          X86_SYNTH(10)) /* SMEP gets used by Xen 
> itself */
>   XEN_CPUFEATURE(XEN_SMAP,          X86_SYNTH(11)) /* SMAP gets used by Xen 
> itself */
>
>



--
Teddy Astie | Vates XCP-ng Developer

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