linux-kernel - Re: [PATCH v9 4/8] x86/cpu, kvm: Move X86_FEATURE_LFENCE

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Date:   Wed, 25 Jan 2023 13:09:33 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Kim Phillips <kim.phillips@....com>, x86@...nel.org,
        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>,
        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: Re: [PATCH v9 4/8] x86/cpu, kvm: Move X86_FEATURE_LFENCE_RDTSC to
 its native leaf

On Tue, Jan 24, 2023 at 09:32:55PM +0000, Sean Christopherson wrote:
> Boris, can you fold this in?

Sure, see below.

---
From: Kim Phillips <kim.phillips@....com>
Date: Tue, 24 Jan 2023 10:33:15 -0600
Subject: [PATCH] x86/cpu, kvm: Move X86_FEATURE_LFENCE_RDTSC to its native leaf

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 in its proper place, the kernel's set_cpu_cap() will
effectively synthesize the feature for KVM going forward.

Also, DE_CFG[1] doesn't need to be set on such CPUs anymore.

  [ bp: Massage and merge diff from Sean. ]

Signed-off-by: Kim Phillips <kim.phillips@....com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Acked-by: Sean Christopherson <seanjc@...gle.com>
Link: https://lore.kernel.org/r/20230124163319.2277355-5-kim.phillips@amd.com
---
 arch/x86/include/asm/cpufeatures.h |  3 ++-
 arch/x86/kernel/cpu/amd.c          |  2 +-
 arch/x86/kvm/cpuid.c               | 16 +++++++++++++---
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 1b2d40a96b97..901128ed4c7a 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 */
@@ -429,6 +429,7 @@
 
 /* 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 aa3a6dc74e95..12455dc5afe5 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -742,12 +742,22 @@ 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 */
 	);
+
+	/*
+	 * 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_caps[CPUID_8000_0021_EAX] |= BIT(2) /* LFENCE Always serializing */;
+		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 */;
-- 
2.35.1

-- 
Regards/Gruss,
    Boris.

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