Re: [PATCH v9 4/8] x86/cpu, kvm: Move X86_FEATURE_LFENCE_RDTSC to its native leaf
From: Sean Christopherson
Date: Tue Jan 24 2023 - 16:33:06 EST
On Tue, Jan 24, 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.
> With LFENCE_RDTSC is in its proper place, the kernel's set_cpu_cap()
> will effectively sythesize the feature for KVM going forward.
>
> 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@xxxxxxx>
> ---
> arch/x86/include/asm/cpufeatures.h | 3 ++-
> arch/x86/kernel/cpu/amd.c | 2 +-
> arch/x86/kvm/cpuid.c | 5 +----
> 3 files changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 4637fd7a84d6..e975822951b2 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 */
> @@ -432,6 +432,7 @@
>
> /* 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 9764499acce2..448b5de98b8f 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -742,12 +742,9 @@ void kvm_set_cpu_caps(void)
> F(SME_COHERENT));
>
> kvm_cpu_cap_mask(CPUID_8000_0021_EAX,
> - F(NO_NESTED_DATA_BP) |
> - BIT(2) /* LFENCE Always serializing */ | 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 */;
Gah, I was wrong. I lost track of the fact that kvm_cpu_cap_mask() does an
actual CPUID, i.e. the oddball code is necessary to manual synthesize the flag.
Boris, can you fold this in?
---
arch/x86/kvm/cpuid.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index fb32e084a40f..12455dc5afe5 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -745,6 +745,19 @@ void kvm_set_cpu_caps(void)
F(NO_NESTED_DATA_BP) | F(LFENCE_RDTSC) | 0 /* SmmPgCfgLock */ |
BIT(6) /* NULL_SEL_CLR_BASE */ | 0 /* PrefetchCtlMsr */
);
+
+ /*
+ * Synthesize "LFENCE is serializing" into the AMD-defined entry in
+ * KVM's supported CPUID if the feature is reported as supported by the
+ * kernel. LFENCE_RDTSC was a Linux-defined synthetic feature long
+ * before AMD joined the bandwagon, e.g. LFENCE is serializing on most
+ * CPUs that support SSE2. On CPUs that don't support AMD's leaf,
+ * kvm_cpu_cap_mask() will unfortunately drop the flag due to ANDing
+ * the mask with the raw host CPUID, and reporting support in AMD's
+ * leaf can make it easier for userspace to detect the feature.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_LFENCE_RDTSC))
+ kvm_cpu_cap_set(X86_FEATURE_LFENCE_RDTSC);
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 */;
base-commit: f607476ee37397a72a2abb687bc170ce0bbec780
--
I.e. end up with this as of this patch:
kvm_cpu_cap_mask(CPUID_8000_0021_EAX,
F(NO_NESTED_DATA_BP) | F(LFENCE_RDTSC) | 0 /* SmmPgCfgLock */ |
BIT(6) /* NULL_SEL_CLR_BASE */ | 0 /* PrefetchCtlMsr */
);
/*
* Synthesize "LFENCE is serializing" into the AMD-defined entry in
* KVM's supported CPUID if the feature is reported as supported by the
* kernel. LFENCE_RDTSC was a Linux-defined synthetic feature long
* before AMD joined the bandwagon, e.g. LFENCE is serializing on most
* CPUs that support SSE2. On CPUs that don't support AMD's leaf,
* kvm_cpu_cap_mask() will unfortunately drop the flag due to ANDing
* the mask with the raw host CPUID, and reporting support in AMD's
* leaf can make it easier for userspace to detect the feature.
*/
if (cpu_feature_enabled(X86_FEATURE_LFENCE_RDTSC))
kvm_cpu_cap_set(X86_FEATURE_LFENCE_RDTSC);
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 */;