[PATCH v8 03/21] KVM: VMX: Disable FRED if FRED consistency checks fail

Xin Li (Intel)
2025-10-14T01:11:00Z

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@intel.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Shan Kang <shan.kang@intel.com>
Tested-by: Xuelian Guo <xuelian.guo@intel.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 6bd67c40ca3b..651507627ef3 100644
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -405,6 +405,16 @@ 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 d881f1c133fa..c6477cb36854 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8001,6 +8001,9 @@ 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();
--
2.51.0