[PATCH 4/4] KVM: arm64: Refuse to run VCPU if the PMU doesn't match the physical CPU

[Alexandru Elisei]

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.

The VCPU is flagged as being scheduled on the wrong CPU in vcpu_load(), but
the flag is cleared when the KVM_RUN enters the non-preemptible section
instead of in vcpu_put(); this has been done on purpose so the error
condition is communicated as soon as possible to userspace, otherwise
vcpu_load() on the wrong CPU followed by a vcpu_put() could clear the flag.

Suggested-by: Marc Zyngier <maz at kernel.org>
Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
---
 Documentation/virt/kvm/api.rst          |  5 +++--
 Documentation/virt/kvm/devices/vcpu.rst |  3 ++-
 arch/arm64/include/asm/kvm_host.h       |  3 +++
 arch/arm64/kvm/arm.c                    | 15 +++++++++++++++
 arch/arm64/kvm/pmu-emul.c               |  1 +
 5 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index aeeb071c7688..5bbad8318ea5 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -396,8 +396,9 @@ Errors:
 
   =======    ==============================================================
   EINTR      an unmasked signal is pending
-  ENOEXEC    the vcpu hasn't been initialized or the guest tried to execute
-             instructions from device memory (arm64)
+  ENOEXEC    the vcpu hasn't been initialized, the guest tried to execute
+             instructions from device memory (arm64) or the vcpu PMU is
+             different from the physical cpu PMU (arm64).
   ENOSYS     data abort outside memslots with no syndrome info and
              KVM_CAP_ARM_NISV_TO_USER not enabled (arm64)
   EPERM      SVE feature set but not finalized (arm64)
diff --git a/Documentation/virt/kvm/devices/vcpu.rst b/Documentation/virt/kvm/devices/vcpu.rst
index 59ac382af59a..ca0da34da889 100644
--- a/Documentation/virt/kvm/devices/vcpu.rst
+++ b/Documentation/virt/kvm/devices/vcpu.rst
@@ -128,7 +128,8 @@ systems where there are at least two PMUs on the system.
 
 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, if the VCPU is scheduled on a CPU which has a different PMU,
+then KVM_RUN will return with the error code ENOEXEC.
 
 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 2a5f7f38006f..ae2083b41d8a 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -385,6 +385,9 @@ struct kvm_vcpu_arch {
 		u64 last_steal;
 		gpa_t base;
 	} steal;
+
+	cpumask_var_t supported_cpus;
+	bool cpu_not_supported;
 };
 
 /* Pointer to the vcpu's SVE FFR for sve_{save,load}_state() */
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 2f03cbfefe67..5dbfd18c4e37 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -320,6 +320,9 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.mmu_page_cache.gfp_zero = __GFP_ZERO;
 
+	if (!zalloc_cpumask_var(&vcpu->arch.supported_cpus, GFP_KERNEL))
+		return -ENOMEM;
+
 	/* Set up the timer */
 	kvm_timer_vcpu_init(vcpu);
 
@@ -347,6 +350,7 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
 	if (vcpu->arch.has_run_once && unlikely(!irqchip_in_kernel(vcpu->kvm)))
 		static_branch_dec(&userspace_irqchip_in_use);
 
+	free_cpumask_var(vcpu->arch.supported_cpus);
 	kvm_mmu_free_memory_cache(&vcpu->arch.mmu_page_cache);
 	kvm_timer_vcpu_terminate(vcpu);
 	kvm_pmu_vcpu_destroy(vcpu);
@@ -425,6 +429,10 @@ 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_empty(vcpu->arch.supported_cpus) &&
+	    !cpumask_test_cpu(smp_processor_id(), vcpu->arch.supported_cpus))
+		vcpu->arch.cpu_not_supported = true;
 }
 
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
@@ -815,6 +823,13 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		 */
 		preempt_disable();
 
+		if (unlikely(vcpu->arch.cpu_not_supported)) {
+			vcpu->arch.cpu_not_supported = false;
+			ret = -ENOEXEC;
+			preempt_enable();
+			continue;
+		}
+
 		kvm_pmu_flush_hwstate(vcpu);
 
 		local_irq_disable();
diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index 53cedeb5dbf6..957a6d0cfa56 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -951,6 +951,7 @@ static int kvm_arm_pmu_v3_set_pmu(struct kvm_vcpu *vcpu, int pmu_id)
 		arm_pmu = entry->arm_pmu;
 		if (arm_pmu->pmu.type == pmu_id) {
 			kvm_pmu->arm_pmu = arm_pmu;
+			cpumask_copy(vcpu->arch.supported_cpus, &arm_pmu->supported_cpus);
 			return 0;
 		}
 	}
-- 
2.33.1