linux-kernel - [PATCH v4 3/8] x86/bugs: Use an X86_FEATURE_xxx flag for the MMIO Stale Data mitigation

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Message-ID: <20251031003040.3491385-4-seanjc@google.com>
Date: Thu, 30 Oct 2025 17:30:35 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Brendan Jackman <jackmanb@...gle.com>
Subject: [PATCH v4 3/8] x86/bugs: Use an X86_FEATURE_xxx flag for the MMIO
 Stale Data mitigation

Convert the MMIO Stale Data mitigation flag from a static branch into an
X86_FEATURE_xxx so that it can be used via ALTERNATIVE_2 in KVM.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/include/asm/cpufeatures.h   |  1 +
 arch/x86/include/asm/nospec-branch.h |  2 --
 arch/x86/kernel/cpu/bugs.c           | 11 +----------
 arch/x86/kvm/mmu/spte.c              |  2 +-
 arch/x86/kvm/vmx/vmx.c               |  4 ++--
 5 files changed, 5 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 7129eb44adad..d1d7b5ec6425 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -501,6 +501,7 @@
 #define X86_FEATURE_ABMC		(21*32+15) /* Assignable Bandwidth Monitoring Counters */
 #define X86_FEATURE_MSR_IMM		(21*32+16) /* MSR immediate form instructions */
 #define X86_FEATURE_X2AVIC_EXT		(21*32+17) /* AMD SVM x2AVIC support for 4k vCPUs */
+#define X86_FEATURE_CLEAR_CPU_BUF_MMIO	(21*32+18) /* Clear CPU buffers using VERW before VMRUN, iff the vCPU can access host MMIO*/
 
 /*
  * BUG word(s)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 923ae21cbef1..b29df45b1edb 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -579,8 +579,6 @@ DECLARE_STATIC_KEY_FALSE(cpu_buf_idle_clear);
 
 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
 
-DECLARE_STATIC_KEY_FALSE(cpu_buf_vm_clear);
-
 extern u16 x86_verw_sel;
 
 #include <asm/segment.h>
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 723666a1357e..9acf6343b0ac 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -192,14 +192,6 @@ EXPORT_SYMBOL_GPL(cpu_buf_idle_clear);
  */
 DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
 
-/*
- * Controls CPU Fill buffer clear before VMenter. This is a subset of
- * X86_FEATURE_CLEAR_CPU_BUF_VM, and should only be enabled when KVM-only
- * mitigation is required.
- */
-DEFINE_STATIC_KEY_FALSE(cpu_buf_vm_clear);
-EXPORT_SYMBOL_GPL(cpu_buf_vm_clear);
-
 #undef pr_fmt
 #define pr_fmt(fmt)	"mitigations: " fmt
 
@@ -751,9 +743,8 @@ static void __init mmio_apply_mitigation(void)
 	if (verw_clear_cpu_buf_mitigation_selected) {
 		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
 		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF_VM);
-		static_branch_disable(&cpu_buf_vm_clear);
 	} else {
-		static_branch_enable(&cpu_buf_vm_clear);
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF_MMIO);
 	}
 
 	/*
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 37647afde7d3..c43dd153d868 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -292,7 +292,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 		mark_page_dirty_in_slot(vcpu->kvm, slot, gfn);
 	}
 
-	if (static_branch_unlikely(&cpu_buf_vm_clear) &&
+	if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF_MMIO) &&
 	    !kvm_vcpu_can_access_host_mmio(vcpu) &&
 	    kvm_is_mmio_pfn(pfn, &is_host_mmio))
 		kvm_track_host_mmio_mapping(vcpu);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 1021d3b65ea0..68cde725d1c7 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -903,7 +903,7 @@ unsigned int __vmx_vcpu_run_flags(struct vcpu_vmx *vmx)
 	if (!msr_write_intercepted(vmx, MSR_IA32_SPEC_CTRL))
 		flags |= VMX_RUN_SAVE_SPEC_CTRL;
 
-	if (static_branch_unlikely(&cpu_buf_vm_clear) &&
+	if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF_MMIO) &&
 	    kvm_vcpu_can_access_host_mmio(&vmx->vcpu))
 		flags |= VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO;
 
@@ -7351,7 +7351,7 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 	 */
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
-	else if (static_branch_unlikely(&cpu_buf_vm_clear) &&
+	else if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF_MMIO) &&
 		 (flags & VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO))
 		x86_clear_cpu_buffers();
 
-- 
2.51.1.930.gacf6e81ea2-goog