linux-kernel - [PATCH v7 4/7] x86/cpu, kvm: Add the Null Selector Clears Base feature

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Message-ID: <20230116230159.1511393-5-kim.phillips@amd.com>
Date:   Mon, 16 Jan 2023 17:01:56 -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 v7 4/7] x86/cpu, kvm: Add the Null Selector Clears Base feature

The Null Selector Clears Base feature was being open-coded for KVM in
__do_cpuid_func().  Add it to its newly added CPUID leaf 0x80000021 EAX
proper, and propagate it in kvm_set_cpu_caps() instead.

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

Signed-off-by: Kim Phillips <kim.phillips@....com>
---
 arch/x86/include/asm/cpufeatures.h |  1 +
 arch/x86/kvm/cpuid.c               | 10 +++-------
 2 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index b22b2e8fef00..ccef41ff718c 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) /* "" AMD 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) /* "" AMD Null Selector Clears Base */
 
 /*
  * BUG word(s)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index d7a13716b7c8..afa86241f752 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -742,10 +742,12 @@ void kvm_set_cpu_caps(void)
 		F(SME_COHERENT));
 
 	kvm_cpu_cap_mask(CPUID_8000_0021_EAX,
-		F(NO_NESTED_DATA_BP) | F(LFENCE_RDTSC)
+		F(NO_NESTED_DATA_BP) | F(LFENCE_RDTSC) | F(NULL_SEL_CLR_BASE)
 	);
 	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_cap_set(X86_FEATURE_NULL_SEL_CLR_BASE);
 
 	kvm_cpu_cap_mask(CPUID_C000_0001_EDX,
 		F(XSTORE) | F(XSTORE_EN) | F(XCRYPT) | F(XCRYPT_EN) |
@@ -1230,9 +1232,6 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		entry->ebx = entry->ecx = entry->edx = 0;
 		cpuid_entry_override(entry, CPUID_8000_0021_EAX);
 		/*
-		 * Pass down these bits:
-		 *    EAX      6      NSCB, Null selector clear base
-		 *
 		 * Other defined bits are for MSRs that KVM does not expose:
 		 *   EAX      3      SPCL, SMM page configuration lock
 		 *   EAX      13     PCMSR, Prefetch control MSR
@@ -1240,10 +1239,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		 * KVM doesn't support SMM_CTL.
 		 *   EAX       9     SMM_CTL MSR is not supported
 		 */
-		entry->eax &= BIT(6);
 		entry->eax |= BIT(9);
-		if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
-			entry->eax |= BIT(6);
 		break;
 	/*Add support for Centaur's CPUID instruction*/
 	case 0xC0000000:
-- 
2.34.1