KVM: arm64: PMU: Reload when user modifies registers

Commit d0c94c4 ("KVM: arm64: Restore PMU configuration on first
run") added the code to reload the PMU configuration on first run.

It is also important to keep the correct state even if system registers
are modified after first run, specifically when debugging Windows on
QEMU with GDB; QEMU tries to write back all visible registers when
resuming the VM execution with GDB, corrupting the PMU state. Windows
always uses the PMU so this can cause adverse effects on that particular
OS.

The usual register writes and reset are already handled independently,
but register writes from userspace are not covered.
Trigger the code to reload the PMU configuration for them instead so
that PMU configuration changes made by users will be applied also after
the first run.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250315-pmc-v5-4-ecee87dab216@daynix.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/pmu-emul.c

@@ -917,9 +917,6 @@ 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;
 }
 

arch/arm64/kvm/sys_regs.c

@@ -1070,6 +1070,8 @@ static int set_pmreg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r, u64 va
 	u64 mask = kvm_pmu_accessible_counter_mask(vcpu);
 
 	__vcpu_sys_reg(vcpu, r->reg) = val & mask;
+	kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+
 	return 0;
 }
 
@@ -1228,6 +1230,8 @@ static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
 		val |= ARMV8_PMU_PMCR_LC;
 
 	__vcpu_sys_reg(vcpu, r->reg) = val;
+	kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+
 	return 0;
 }