KVM: arm64: Refuse to run VCPU if the PMU doesn't match the physical CPU

Userspace can assign a PMU to a VCPU with the KVM_ARM_VCPU_PMU_V3_SET_PMU
device ioctl. If the VCPU is scheduled on a physical CPU which has a
different PMU, the perf events needed to emulate a guest PMU won't be
scheduled in and the guest performance counters will stop counting. Treat
it as an userspace error and refuse to run the VCPU in this situation.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220127161759.53553-7-alexandru.elisei@arm.com

5 files changed, 38 insertions, 1 deletions

diff --git a/Documentation/virt/kvm/devices/vcpu.rst b/Documentation/virt/kvm/devices/vcpu.rst
index e8c5770590a2..260db203a1e2 100644
--- a/Documentation/virt/kvm/devices/vcpu.rst
+++ b/Documentation/virt/kvm/devices/vcpu.rst
@@ -131,7 +131,11 @@ if a PMU event filter is already present.
 
 Note that KVM will not make any attempts to run the VCPU on the physical CPUs
 associated with the PMU specified by this attribute. This is entirely left to
-userspace.
+userspace. However, attempting to run the VCPU on a physical CPU not supported
+by the PMU will fail and KVM_RUN will return with
+exit_reason = KVM_EXIT_FAIL_ENTRY and populate the fail_entry struct by setting
+hardare_entry_failure_reason field to KVM_EXIT_FAIL_ENTRY_CPU_UNSUPPORTED and
+the cpu field to the processor id.
 
 2. GROUP: KVM_ARM_VCPU_TIMER_CTRL
 =================================
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 57141a3a3740..0bed0e33e1ae 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -131,6 +131,8 @@ struct kvm_arch {
 	unsigned long *pmu_filter;
 	struct arm_pmu *arm_pmu;
 
+	cpumask_var_t supported_cpus;
+
 	u8 pfr0_csv2;
 	u8 pfr0_csv3;
 
@@ -436,6 +438,7 @@ struct kvm_vcpu_arch {
 #define KVM_ARM64_DEBUG_STATE_SAVE_SPE	(1 << 12) /* Save SPE context if active  */
 #define KVM_ARM64_DEBUG_STATE_SAVE_TRBE	(1 << 13) /* Save TRBE context if active  */
 #define KVM_ARM64_FP_FOREIGN_FPSTATE	(1 << 14)
+#define KVM_ARM64_ON_UNSUPPORTED_CPU	(1 << 15) /* Physical CPU not in supported_cpus */
 
 #define KVM_GUESTDBG_VALID_MASK (KVM_GUESTDBG_ENABLE | \
 				 KVM_GUESTDBG_USE_SW_BP | \
@@ -454,6 +457,15 @@ struct kvm_vcpu_arch {
 #define vcpu_has_ptrauth(vcpu)		false
 #endif
 
+#define vcpu_on_unsupported_cpu(vcpu)					\
+	((vcpu)->arch.flags & KVM_ARM64_ON_UNSUPPORTED_CPU)
+
+#define vcpu_set_on_unsupported_cpu(vcpu)				\
+	((vcpu)->arch.flags |= KVM_ARM64_ON_UNSUPPORTED_CPU)
+
+#define vcpu_clear_on_unsupported_cpu(vcpu)				\
+	((vcpu)->arch.flags &= ~KVM_ARM64_ON_UNSUPPORTED_CPU)
+
 #define vcpu_gp_regs(v)		(&(v)->arch.ctxt.regs)
 
 /*
diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index 1d0a0a2a9711..d49f714f48e6 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -414,6 +414,9 @@ struct kvm_arm_copy_mte_tags {
 #define KVM_PSCI_RET_INVAL		PSCI_RET_INVALID_PARAMS
 #define KVM_PSCI_RET_DENIED		PSCI_RET_DENIED
 
+/* run->fail_entry.hardware_entry_failure_reason codes. */
+#define KVM_EXIT_FAIL_ENTRY_CPU_UNSUPPORTED	(1ULL << 0)
+
 #endif
 
 #endif /* __ARM_KVM_H__ */
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 4783dbf66df2..13c1318d8b9a 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -150,6 +150,10 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	if (ret)
 		goto out_free_stage2_pgd;
 
+	if (!zalloc_cpumask_var(&kvm->arch.supported_cpus, GFP_KERNEL))
+		goto out_free_stage2_pgd;
+	cpumask_copy(kvm->arch.supported_cpus, cpu_possible_mask);
+
 	kvm_vgic_early_init(kvm);
 
 	/* The maximum number of VCPUs is limited by the host's GIC model */
@@ -176,6 +180,7 @@ vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
 void kvm_arch_destroy_vm(struct kvm *kvm)
 {
 	bitmap_free(kvm->arch.pmu_filter);
+	free_cpumask_var(kvm->arch.supported_cpus);
 
 	kvm_vgic_destroy(kvm);
 
@@ -411,6 +416,9 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	if (vcpu_has_ptrauth(vcpu))
 		vcpu_ptrauth_disable(vcpu);
 	kvm_arch_vcpu_load_debug_state_flags(vcpu);
+
+	if (!cpumask_test_cpu(smp_processor_id(), vcpu->kvm->arch.supported_cpus))
+		vcpu_set_on_unsupported_cpu(vcpu);
 }
 
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
@@ -423,6 +431,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 	kvm_vgic_put(vcpu);
 	kvm_vcpu_pmu_restore_host(vcpu);
 
+	vcpu_clear_on_unsupported_cpu(vcpu);
 	vcpu->cpu = -1;
 }
 
@@ -796,6 +805,14 @@ static bool kvm_vcpu_exit_request(struct kvm_vcpu *vcpu, int *ret)
 		}
 	}
 
+	if (unlikely(vcpu_on_unsupported_cpu(vcpu))) {
+		run->exit_reason = KVM_EXIT_FAIL_ENTRY;
+		run->fail_entry.hardware_entry_failure_reason = KVM_EXIT_FAIL_ENTRY_CPU_UNSUPPORTED;
+		run->fail_entry.cpu = smp_processor_id();
+		*ret = 0;
+		return true;
+	}
+
 	return kvm_request_pending(vcpu) ||
 			need_new_vmid_gen(&vcpu->arch.hw_mmu->vmid) ||
 			xfer_to_guest_mode_work_pending();
diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index 18361f367495..4526a5824dac 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -968,6 +968,7 @@ static int kvm_arm_pmu_v3_set_pmu(struct kvm_vcpu *vcpu, int pmu_id)
 			}
 
 			kvm->arch.arm_pmu = arm_pmu;
+			cpumask_copy(kvm->arch.supported_cpus, &arm_pmu->supported_cpus);
 			ret = 0;
 			break;
 		}