linux-kernel - [RFC PATCH v2 03/11] KVM: x86: Advertise BHI

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Date:   Fri, 14 Apr 2023 14:25:24 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     kvm@...r.kernel.org
Cc:     Jiaan Lu <jiaan.lu@...el.com>, Zhang Chen <chen.zhang@...el.com>,
        Chao Gao <chao.gao@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: [RFC PATCH v2 03/11] KVM: x86: Advertise BHI_CTRL support

From: Zhang Chen <chen.zhang@...el.com>

Add 100% kvm-only feature for BHI_CTRL because the kernel doesn't use it
at all.

BHI_CTRL is enumerated by CPUID.7.2.EDX[4]. If supported, BHI_DIS_S (bit
10) of IA32_SPEC_CTRL MSR can be used to enable BHI_DIS_S behavior.

Note that KVM does not intercept guests' IA32_SPEC_CTRL MSR accesses
after a non-zero is written to the MSR. Therefore, guests can already
toggle the BHI_DIS_S bit if the host supports BHI_CTRL, and no extra
code is needed to allow guests to toggle the bit.

Signed-off-by: Zhang Chen <chen.zhang@...el.com>
Signed-off-by: Chao Gao <chao.gao@...el.com>
Tested-by: Jiaan Lu <jiaan.lu@...el.com>
---
 arch/x86/kvm/cpuid.c         | 2 +-
 arch/x86/kvm/reverse_cpuid.h | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index f024c3ac2203..7cdd859d09a2 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -686,7 +686,7 @@ void kvm_set_cpu_caps(void)
 	);
 
 	kvm_cpu_cap_init_kvm_defined(CPUID_7_2_EDX,
-		SF(RRSBA_CTRL)
+		SF(RRSBA_CTRL) | F(BHI_CTRL)
 	);
 
 	kvm_cpu_cap_mask(CPUID_8000_0001_ECX,
diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
index 72bad8314a9c..e7e70c9aa384 100644
--- a/arch/x86/kvm/reverse_cpuid.h
+++ b/arch/x86/kvm/reverse_cpuid.h
@@ -50,6 +50,7 @@ enum kvm_only_cpuid_leafs {
 
 /* Intel-defined sub-features, CPUID level 0x00000007:2 (EDX) */
 #define KVM_X86_FEATURE_RRSBA_CTRL	KVM_X86_FEATURE(CPUID_7_2_EDX, 2)
+#define X86_FEATURE_BHI_CTRL		KVM_X86_FEATURE(CPUID_7_2_EDX, 4)
 
 struct cpuid_reg {
 	u32 function;
-- 
2.40.0