Re: [PATCH v8 4/8] x86/cpu, kvm: Move X86_FEATURE_LFENCE_RDTSC to its native leaf

Sean Christopherson
2023-01-24T01:18:00Z

On Mon, Jan 23, 2023, Kim Phillips wrote:
> The LFENCE always serializing feature bit was defined as scattered
> LFENCE_RDTSC and its native leaf bit position open-coded for KVM.
> Add it to its newly added CPUID leaf 0x80000021 EAX proper.
>
> Drop the bit description comments now it's more self-describing.
>
> Also, in amd_init(), don't bother setting DE_CFG[1] any more.
>
> Signed-off-by: Kim Phillips <kim.phillips@amd.com>
> ---
> arch/x86/include/asm/cpufeatures.h | 3 ++-
> arch/x86/kernel/cpu/amd.c | 2 +-
> arch/x86/kvm/cpuid.c | 4 ++--
> 3 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 7f0fb894e432..4f22d828c753 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -97,7 +97,7 @@
> #define X86_FEATURE_SYSENTER32 ( 3*32+15) /* "" sysenter in IA32 userspace */
> #define X86_FEATURE_REP_GOOD ( 3*32+16) /* REP microcode works well */
> #define X86_FEATURE_AMD_LBR_V2 ( 3*32+17) /* AMD Last Branch Record Extension Version 2 */
> -#define X86_FEATURE_LFENCE_RDTSC ( 3*32+18) /* "" LFENCE synchronizes RDTSC */
> +/* FREE, was #define X86_FEATURE_LFENCE_RDTSC ( 3*32+18) "" LFENCE synchronizes RDTSC */
> #define X86_FEATURE_ACC_POWER ( 3*32+19) /* AMD Accumulated Power Mechanism */
> #define X86_FEATURE_NOPL ( 3*32+20) /* The NOPL (0F 1F) instructions */
> #define X86_FEATURE_ALWAYS ( 3*32+21) /* "" Always-present feature */
> @@ -430,6 +430,7 @@
>
> /* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */
> #define X86_FEATURE_NO_NESTED_DATA_BP (20*32+ 0) /* "" No Nested Data Breakpoints */
> +#define X86_FEATURE_LFENCE_RDTSC (20*32+ 2) /* "" LFENCE always serializing / synchronizes RDTSC */
>
> /*
> * BUG word(s)
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index f769d6d08b43..208c2ce8598a 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -956,7 +956,7 @@ static void init_amd(struct cpuinfo_x86 *c)
>
> init_amd_cacheinfo(c);
>
> - if (cpu_has(c, X86_FEATURE_XMM2)) {
> + if (!cpu_has(c, X86_FEATURE_LFENCE_RDTSC) && cpu_has(c, X86_FEATURE_XMM2)) {
> /*
> * Use LFENCE for execution serialization. On families which
> * don't have that MSR, LFENCE is already serializing.
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 13bd2769fa5a..601eeb03ebc9 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -742,11 +742,11 @@ void kvm_set_cpu_caps(void)
> F(SME_COHERENT));
>
> kvm_cpu_cap_mask(CPUID_8000_0021_EAX,
> - F(NO_NESTED_DATA_BP) | 0 /* SmmPgCfgLock */ |
> + F(NO_NESTED_DATA_BP) | F(LFENCE_RDTSC) | 0 /* SmmPgCfgLock */ |
> BIT(6) /* NULL_SEL_CLR_BASE */ | 0 /* PrefetchCtlMsr */
> );
> if (cpu_feature_enabled(X86_FEATURE_LFENCE_RDTSC))
> - kvm_cpu_caps[CPUID_8000_0021_EAX] |= BIT(2) /* LFENCE Always serializing */;
> + kvm_cpu_cap_set(X86_FEATURE_LFENCE_RDTSC);

Now that LFENCE_RDTSC is in its proper place, the kernel's set_cpu_cap() will
effectively sythesize the feature for KVM. I.e. this patch can simply delete the
synthesis of the feature.

> if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
> kvm_cpu_caps[CPUID_8000_0021_EAX] |= BIT(6) /* NULL_SEL_CLR_BASE */;
> kvm_cpu_caps[CPUID_8000_0021_EAX] |= BIT(9) /* NO_SMM_CTL_MSR */;
> --
> 2.34.1
>