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

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Date:   Mon, 23 Jan 2023 16:56:57 -0600
From:   Kim Phillips <kim.phillips@....com>
To:     <x86@...nel.org>
CC:     Kim Phillips <kim.phillips@....com>,
        Borislav Petkov <bp@...en8.de>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Alexey Kardashevskiy" <aik@....com>, <kvm@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH v8 5/8] x86/cpu, kvm: Add the Null Selector Clears Base feature

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@....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