linux-kernel - [PATCH v6 03/20] KVM: VMX: Disable FRED if FRED consistency checks fail

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Message-ID: <20250821223630.984383-4-xin@zytor.com>
Date: Thu, 21 Aug 2025 15:36:12 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc: pbonzini@...hat.com, seanjc@...gle.com, corbet@....net, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, xin@...or.com, luto@...nel.org,
        peterz@...radead.org, andrew.cooper3@...rix.com, chao.gao@...el.com,
        hch@...radead.org
Subject: [PATCH v6 03/20] KVM: VMX: Disable FRED if FRED consistency checks fail

From: Xin Li <xin3.li@...el.com>

Do not virtualize FRED if FRED consistency checks fail.

Either on broken hardware, or when run KVM on top of another hypervisor
before the underlying hypervisor implements nested FRED correctly.

Suggested-by: Chao Gao <chao.gao@...el.com>
Signed-off-by: Xin Li <xin3.li@...el.com>
Signed-off-by: Xin Li (Intel) <xin@...or.com>
Reviewed-by: Chao Gao <chao.gao@...el.com>
Tested-by: Shan Kang <shan.kang@...el.com>
Tested-by: Xuelian Guo <xuelian.guo@...el.com>
---

Changes in v5:
* Drop the cpu_feature_enabled() in cpu_has_vmx_fred() (Sean).
* Add TB from Xuelian Guo.

Change in v4:
* Call out the reason why not check FRED VM-exit controls in
  cpu_has_vmx_fred() (Chao Gao).
---
 arch/x86/kvm/vmx/capabilities.h | 10 ++++++++++
 arch/x86/kvm/vmx/vmx.c          |  3 +++
 2 files changed, 13 insertions(+)

diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
index 7b9e306c359d..7fe95a601c9f 100644
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -408,6 +408,16 @@ static inline bool vmx_pebs_supported(void)
 	return boot_cpu_has(X86_FEATURE_PEBS) && kvm_pmu_cap.pebs_ept;
 }
 
+static inline bool cpu_has_vmx_fred(void)
+{
+	/*
+	 * setup_vmcs_config() guarantees FRED VM-entry/exit controls
+	 * are either all set or none.  So, no need to check FRED VM-exit
+	 * controls.
+	 */
+	return (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_FRED);
+}
+
 static inline bool cpu_has_notify_vmexit(void)
 {
 	return vmcs_config.cpu_based_2nd_exec_ctrl &
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3b5e2805a06d..c8b95c215869 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7993,6 +7993,9 @@ static __init void vmx_set_cpu_caps(void)
 		kvm_cpu_cap_check_and_set(X86_FEATURE_DTES64);
 	}
 
+	if (!cpu_has_vmx_fred())
+		kvm_cpu_cap_clear(X86_FEATURE_FRED);
+
 	if (!enable_pmu)
 		kvm_cpu_cap_clear(X86_FEATURE_PDCM);
 	kvm_caps.supported_perf_cap = vmx_get_perf_capabilities();
-- 
2.50.1