KVM: arm64: Restore PMU configuration on first run

Restoring a guest with an active virtual PMU results in no perf
counters being instanciated on the host side. Not quite what
you'd expect from a restore.

In order to fix this, force a writeback of PMCR_EL0 on the first
run of a vcpu (using a new request so that it happens once the
vcpu has been loaded). This will in turn create all the host-side
counters that were missing.
Reported-by: Jinank Jain <jinankj@amazon.de>
Tested-by: Jinank Jain <jinankj@amazon.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/87wnrbylxv.wl-maz@kernel.org
Link: https://lore.kernel.org/r/b53dfcf9bbc4db7f96154b1cd5188d72b9766358.camel@amazon.de

arch/arm64/include/asm/kvm_host.h

@@ -46,6 +46,7 @@
 #define KVM_REQ_VCPU_RESET	KVM_ARCH_REQ(2)
 #define KVM_REQ_RECORD_STEAL	KVM_ARCH_REQ(3)
 #define KVM_REQ_RELOAD_GICv4	KVM_ARCH_REQ(4)
+#define KVM_REQ_RELOAD_PMU	KVM_ARCH_REQ(5)
 
 #define KVM_DIRTY_LOG_MANUAL_CAPS   (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
 				     KVM_DIRTY_LOG_INITIALLY_SET)

arch/arm64/kvm/arm.c

@@ -689,6 +689,10 @@ static void check_vcpu_requests(struct kvm_vcpu *vcpu)
 			vgic_v4_load(vcpu);
 			preempt_enable();
 		}
+
+		if (kvm_check_request(KVM_REQ_RELOAD_PMU, vcpu))
+			kvm_pmu_handle_pmcr(vcpu,
+					    __vcpu_sys_reg(vcpu, PMCR_EL0));
 	}
 }
 

arch/arm64/kvm/pmu-emul.c

@@ -851,6 +851,9 @@ int kvm_arm_pmu_v3_enable(struct kvm_vcpu *vcpu)
 		   return -EINVAL;
 	}
 
+	/* One-off reload of the PMU on first run */
+	kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+
 	return 0;
 }