[PATCH v4 1/1] KVM: x86: Introduce has_protected_pmu state for TDX VMs

[PATCH v4 1/1] KVM: x86: Introduce has_protected_pmu state for TDX VMs

[Vishal Annapurve]

PMU state for TDX VMs is virtualized by TDX Module [1]. Host has following
toggles to control the PMU functionality exposed to TDX VMs:
1) Configure TD_PARAMS to allow guests to use performance monitoring.
2) Restrict the TD to a subset of the PEBS counters if supported.
3) Limit the TD to setup a certain perfmon events using basic/enhanced
   event filtering.

KVM will need to be enlightened to support these toggles. Introduce
has_protected_pmu state to track the pmu state for such scenarios
and explicitly set the has_protected_pmu flag for TDX VMs.

If pmu state is protected:
1) Disable KVM's PMU virtualization framework as additional
enlightenment is needed within KVM to control/manage the
visibility of PMU state to such VMs.
2) Disallow userspace VMM from enabling PMU virtualization state
using KVM_CAP_PMU_CAPABILITY.

[1] Section 15.2: https://cdrdv2.intel.com/v1/dl/getContent/733575

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vishal Annapurve <vannapurve@google.com>
---
v3 -> v4:
 - Allow userspace to disable pmu virtualization even if
   has_protected_pmu is set.
 - Addressed feedback around enable_pmu initialization in
   kvm_arch_init_vm().

v3: https://lore.kernel.org/kvm/20260507142402.2175933-1-vannapurve@google.com/

 arch/x86/include/asm/kvm_host.h | 1 +
 arch/x86/kvm/vmx/tdx.c          | 6 ++++++
 arch/x86/kvm/x86.c              | 9 ++++++++-
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c470e40a00aa..8371dcaaed1a 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1422,6 +1422,7 @@ struct kvm_arch {
 	bool has_private_mem;
 	bool has_protected_state;
 	bool has_protected_eoi;
+	bool has_protected_pmu;
 	bool pre_fault_allowed;
 	struct hlist_head *mmu_page_hash;
 	struct list_head active_mmu_pages;
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 1e47c194af53..eb4b4518e6f0 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -638,6 +638,12 @@ int tdx_vm_init(struct kvm *kvm)
 	kvm->arch.has_private_mem = true;
 	kvm->arch.disabled_quirks |= KVM_X86_QUIRK_IGNORE_GUEST_PAT;
 
+	/*
+	 * PMU support is provided by the TDX-Module (if enabled for the VM).
+	 * From KVM's perspective, the VM doesn't have a virtual PMU.
+	 */
+	kvm->arch.has_protected_pmu = true;
+
 	/*
 	 * Because guest TD is protected, VMM can't parse the instruction in TD.
 	 * Instead, guest uses MMIO hypercall.  For unmodified device driver,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0a1b63c63d1a..c248abb0e2ab 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6909,6 +6909,13 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		if (!enable_pmu || (cap->args[0] & ~KVM_CAP_PMU_VALID_MASK))
 			break;
 
+		if (kvm->arch.has_protected_pmu) {
+			WARN_ON_ONCE(kvm->arch.enable_pmu);
+			if (cap->args[0] == KVM_PMU_CAP_DISABLE)
+				r = 0;
+			break;
+		}
+
 		mutex_lock(&kvm->lock);
 		if (!kvm->created_vcpus && !kvm->arch.created_mediated_pmu) {
 			kvm->arch.enable_pmu = !(cap->args[0] & KVM_PMU_CAP_DISABLE);
@@ -13375,7 +13382,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	kvm->arch.default_tsc_khz = max_tsc_khz ? : tsc_khz;
 	kvm->arch.apic_bus_cycle_ns = APIC_BUS_CYCLE_NS_DEFAULT;
 	kvm->arch.guest_can_read_msr_platform_info = true;
-	kvm->arch.enable_pmu = enable_pmu;
+	kvm->arch.enable_pmu = enable_pmu && !kvm->arch.has_protected_pmu;
 
 #if IS_ENABLED(CONFIG_HYPERV)
 	spin_lock_init(&kvm->arch.hv_root_tdp_lock);
-- 
2.54.0.563.g4f69b47b94-goog