KVM: VMX: Add mediated PMU support for CPUs without "save perf global ctrl"

Extend mediated PMU support for Intel CPUs without support for saving
PERF_GLOBAL_CONTROL into the guest VMCS field on VM-Exit, e.g. for Skylake
and its derivatives, as well as Icelake.  While supporting CPUs without
VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL isn't completely trivial, it's not that
complex either.  And not supporting such CPUs would mean not supporting 7+
years of Intel CPUs released in the past 10 years.

On VM-Exit, immediately propagate the saved PERF_GLOBAL_CTRL to the VMCS
as well as KVM's software cache so that KVM doesn't need to add full EXREG
tracking of PERF_GLOBAL_CTRL.  In practice, the vast majority of VM-Exits
won't trigger software writes to guest PERF_GLOBAL_CTRL, so deferring the
VMWRITE to the next VM-Enter would only delay the inevitable without
batching/avoiding VMWRITEs.

Note!  Take care to refresh VM_EXIT_MSR_STORE_COUNT on nested VM-Exit, as
it's unfortunately possible that KVM could recalculate MSR intercepts
while L2 is active, e.g. if userspace loads nested state and _then_ sets
PERF_CAPABILITIES.  Eating the VMWRITE on every nested VM-Exit is
unfortunate, but that's a pre-existing problem and can/should be solved
separately, e.g. modifying the number of auto-load entries while L2 is
active is also uncommon on modern CPUs.

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-45-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/nested.c

@@ -5142,7 +5142,11 @@ void __nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 
 	kvm_nested_vmexit_handle_ibrs(vcpu);
 
-	/* Update any VMCS fields that might have changed while L2 ran */
+	/*
+	 * Update any VMCS fields that might have changed while vmcs02 was the
+	 * active VMCS.  The tracking is per-vCPU, not per-VMCS.
+	 */
+	vmcs_write32(VM_EXIT_MSR_STORE_COUNT, vmx->msr_autostore.nr);
 	vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, vmx->msr_autoload.host.nr);
 	vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_autoload.guest.nr);
 	vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);

arch/x86/kvm/vmx/pmu_intel.c

@@ -777,13 +777,6 @@ static bool intel_pmu_is_mediated_pmu_supported(struct x86_pmu_capability *host_
 	if (WARN_ON_ONCE(!cpu_has_load_perf_global_ctrl()))
 		return false;
 
-	/*
-	 * KVM doesn't yet support mediated PMU on CPUs without support for
-	 * saving PERF_GLOBAL_CTRL via a dedicated VMCS field.
-	 */
-	if (!cpu_has_save_perf_global_ctrl())
-		return false;
-
 	return true;
 }
 

arch/x86/kvm/vmx/vmx.c

@@ -1204,6 +1204,17 @@ static bool update_transition_efer(struct vcpu_vmx *vmx)
 	return true;
 }
 
+static void vmx_add_autostore_msr(struct vcpu_vmx *vmx, u32 msr)
+{
+	vmx_add_auto_msr(&vmx->msr_autostore, msr, 0, VM_EXIT_MSR_STORE_COUNT,
+			 vmx->vcpu.kvm);
+}
+
+static void vmx_remove_autostore_msr(struct vcpu_vmx *vmx, u32 msr)
+{
+	vmx_remove_auto_msr(&vmx->msr_autostore, msr, VM_EXIT_MSR_STORE_COUNT);
+}
+
 #ifdef CONFIG_X86_32
 /*
  * On 32-bit kernels, VM exits still load the FS and GS bases from the
@@ -4225,6 +4236,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
 
 static void vmx_recalc_pmu_msr_intercepts(struct kvm_vcpu *vcpu)
 {
+	u64 vm_exit_controls_bits = VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL |
+				    VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL;
 	bool has_mediated_pmu = kvm_vcpu_has_mediated_pmu(vcpu);
 	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -4234,12 +4247,19 @@ static void vmx_recalc_pmu_msr_intercepts(struct kvm_vcpu *vcpu)
 	if (!enable_mediated_pmu)
 		return;
 
+	if (!cpu_has_save_perf_global_ctrl()) {
+		vm_exit_controls_bits &= ~VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL;
+
+		if (has_mediated_pmu)
+			vmx_add_autostore_msr(vmx, MSR_CORE_PERF_GLOBAL_CTRL);
+		else
+			vmx_remove_autostore_msr(vmx, MSR_CORE_PERF_GLOBAL_CTRL);
+	}
+
 	vm_entry_controls_changebit(vmx, VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL,
 				    has_mediated_pmu);
 
-	vm_exit_controls_changebit(vmx, VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL |
-					VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL,
-				   has_mediated_pmu);
+	vm_exit_controls_changebit(vmx, vm_exit_controls_bits, has_mediated_pmu);
 
 	for (i = 0; i < pmu->nr_arch_gp_counters; i++) {
 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PERFCTR0 + i,
@@ -7346,6 +7366,29 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
 					      msrs[i].host);
 }
 
+static void vmx_refresh_guest_perf_global_control(struct kvm_vcpu *vcpu)
+{
+	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
+	struct vcpu_vmx *vmx = to_vmx(vcpu);
+
+	if (msr_write_intercepted(vmx, MSR_CORE_PERF_GLOBAL_CTRL))
+		return;
+
+	if (!cpu_has_save_perf_global_ctrl()) {
+		int slot = vmx_find_loadstore_msr_slot(&vmx->msr_autostore,
+						       MSR_CORE_PERF_GLOBAL_CTRL);
+
+		if (WARN_ON_ONCE(slot < 0))
+			return;
+
+		pmu->global_ctrl = vmx->msr_autostore.val[slot].value;
+		vmcs_write64(GUEST_IA32_PERF_GLOBAL_CTRL, pmu->global_ctrl);
+		return;
+	}
+
+	pmu->global_ctrl = vmcs_read64(GUEST_IA32_PERF_GLOBAL_CTRL);
+}
+
 static void vmx_update_hv_timer(struct kvm_vcpu *vcpu, bool force_immediate_exit)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -7631,8 +7674,7 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
 
 	vmx->loaded_vmcs->launched = 1;
 
-	if (!msr_write_intercepted(vmx, MSR_CORE_PERF_GLOBAL_CTRL))
-		vcpu_to_pmu(vcpu)->global_ctrl = vmcs_read64(GUEST_IA32_PERF_GLOBAL_CTRL);
+	vmx_refresh_guest_perf_global_control(vcpu);
 
 	vmx_recover_nmi_blocking(vmx);
 	vmx_complete_interrupts(vmx);