linux-kernel - [PATCH v2] x86/bugs/mmio: Rename mmio_stale_data_clear to cpu_buf_vm

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Message-ID: <20250416-mmio-rename-v2-1-ad1f5488767c@linux.intel.com>
Date: Wed, 16 Apr 2025 06:47:51 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: [PATCH v2] x86/bugs/mmio: Rename mmio_stale_data_clear to
 cpu_buf_vm_clear

The static key mmio_stale_data_clear controls the KVM-only mitigation for
MMIO Stale Data vulnerability. Rename it to reflect its purpose.

No functional change.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
---
v2:
- Clarify the case when cpu_buf_vm_clear is used. (Sean)
---
 arch/x86/include/asm/nospec-branch.h |  2 +-
 arch/x86/kernel/cpu/bugs.c           | 16 ++++++++++------
 arch/x86/kvm/vmx/vmx.c               |  6 +++++-
 3 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 5c43f145454ddda781e21806752737b42789c86d..81c4a13e4c0d4ace02b0948eef79377a68252e63 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -561,7 +561,7 @@ DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
 
 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
 
-DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear);
+DECLARE_STATIC_KEY_FALSE(cpu_buf_vm_clear);
 
 extern u16 mds_verw_sel;
 
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 362602b705cc43bd5e9df7c2157f44e7bfb304b9..9131e612de170a6b2bf27c35651f8e824f3d016c 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -127,9 +127,13 @@ EXPORT_SYMBOL_GPL(mds_idle_clear);
  */
 DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
 
-/* Controls CPU Fill buffer clear before KVM guest MMIO accesses */
-DEFINE_STATIC_KEY_FALSE(mmio_stale_data_clear);
-EXPORT_SYMBOL_GPL(mmio_stale_data_clear);
+/*
+ * Controls CPU Fill buffer clear before VMenter. This is a subset of
+ * X86_FEATURE_CLEAR_CPU_BUF, 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);
 
 void __init cpu_select_mitigations(void)
 {
@@ -449,9 +453,9 @@ static void __init mmio_select_mitigation(void)
 	 * mitigations, disable KVM-only mitigation in that case.
 	 */
 	if (boot_cpu_has(X86_FEATURE_CLEAR_CPU_BUF))
-		static_branch_disable(&mmio_stale_data_clear);
+		static_branch_disable(&cpu_buf_vm_clear);
 	else
-		static_branch_enable(&mmio_stale_data_clear);
+		static_branch_enable(&cpu_buf_vm_clear);
 
 	/*
 	 * If Processor-MMIO-Stale-Data bug is present and Fill Buffer data can
@@ -571,7 +575,7 @@ static void __init md_clear_update_mitigation(void)
 		taa_select_mitigation();
 	}
 	/*
-	 * MMIO_MITIGATION_OFF is not checked here so that mmio_stale_data_clear
+	 * MMIO_MITIGATION_OFF is not checked here so that cpu_buf_vm_clear
 	 * gets updated correctly as per X86_FEATURE_CLEAR_CPU_BUF state.
 	 */
 	if (boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 5c5766467a61d434ba2baa79a5faba99bcbd9997..a1754f7ba889853eca919f090794b0dc54229a1a 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7358,10 +7358,14 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 	 * mitigation for MDS is done late in VMentry and is still
 	 * executed in spite of L1D Flush. This is because an extra VERW
 	 * should not matter much after the big hammer L1D Flush.
+	 *
+	 * cpu_buf_vm_clear is used when system is not vulnerable to MDS/TAA,
+	 * and is affected by MMIO Stale Data. In such cases mitigation in only
+	 * needed against an MMIO capable guest.
 	 */
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
-	else if (static_branch_unlikely(&mmio_stale_data_clear) &&
+	else if (static_branch_unlikely(&cpu_buf_vm_clear) &&
 		 kvm_arch_has_assigned_device(vcpu->kvm))
 		mds_clear_cpu_buffers();
 

---
base-commit: 8ffd015db85fea3e15a77027fda6c02ced4d2444
change-id: 20250410-mmio-rename-cf8acecb563e

Best regards,
-- 
Pawan