KVM: arm64: PMU: Don't save PMCR_EL0.{C,P} for the vCPU

Presently, when a guest writes 1 to PMCR_EL0.{C,P}, which is WO/RAZ,
KVM saves the register value, including these bits.
When userspace reads the register using KVM_GET_ONE_REG, KVM returns
the saved register value as it is (the saved value might have these
bits set).  This could result in userspace setting these bits on the
destination during migration.  Consequently, KVM may end up resetting
the vPMU counter registers (PMCCNTR_EL0 and/or PMEVCNTR<n>_EL0) to
zero on the first KVM_RUN after migration.

Fix this by not saving those bits when a guest writes 1 to those bits.

Fixes: ab946834 ("arm64: KVM: Add access handler for PMCR register")
Cc: stable@vger.kernel.org
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Reiji Watanabe <reijiw@google.com>
Link: https://lore.kernel.org/r/20230313033234.1475987-1-reijiw@google.comSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/pmu-emul.c

@@ -538,7 +538,8 @@ void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
 	if (!kvm_pmu_is_3p5(vcpu))
 		val &= ~ARMV8_PMU_PMCR_LP;
 
-	__vcpu_sys_reg(vcpu, PMCR_EL0) = val;
+	/* The reset bits don't indicate any state, and shouldn't be saved. */
+	__vcpu_sys_reg(vcpu, PMCR_EL0) = val & ~(ARMV8_PMU_PMCR_C | ARMV8_PMU_PMCR_P);
 
 	if (val & ARMV8_PMU_PMCR_E) {
 		kvm_pmu_enable_counter_mask(vcpu,