[PATCH v8 5/8] x86/cpu, kvm: Add the Null Selector Clears Base feature

Kim Phillips
2023-01-23T22:58:32Z

The Null Selector Clears Base feature was being open-coded for KVM.
Add it to its newly added native CPUID leaf 0x80000021 EAX proper.

Also drop the bit description comments now it's more self-describing.

[ bp: Convert test in check_null_seg_clears_base() too. ]

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/kernel/cpu/common.c | 4 +---
arch/x86/kvm/cpuid.c | 4 ++--
3 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 4f22d828c753..403a534691cc 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -431,6 +431,7 @@
/* 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 */
+#define X86_FEATURE_NULL_SEL_CLR_BASE (20*32+ 6) /* "" Null Selector Clears Base */

/*
* BUG word(s)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 094dbcd63f2a..162352d42ce0 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1685,9 +1685,7 @@ void check_null_seg_clears_base(struct cpuinfo_x86 *c)
if (!IS_ENABLED(CONFIG_X86_64))
return;

- /* Zen3 CPUs advertise Null Selector Clears Base in CPUID. */
- if (c->extended_cpuid_level >= 0x80000021 &&
- cpuid_eax(0x80000021) & BIT(6))
+ if (cpu_has(c, X86_FEATURE_NULL_SEL_CLR_BASE))
return;

/*
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 601eeb03ebc9..f1625a58b5ec 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -743,12 +743,12 @@ void kvm_set_cpu_caps(void)

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 */
+ F(NULL_SEL_CLR_BASE) | 0 /* PrefetchCtlMsr */
);
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_cap_set(X86_FEATURE_NULL_SEL_CLR_BASE);
kvm_cpu_caps[CPUID_8000_0021_EAX] |= BIT(9) /* NO_SMM_CTL_MSR */;

kvm_cpu_cap_mask(CPUID_C000_0001_EDX,
--
2.34.1