KVM: arm64: Correctly handle RES0 bits PMEVTYPER<n>_EL0.evtCount

The PMU event ID varies from 10 to 16 bits, depending on the PMU
version. If the PMU only supports 10 bits of event ID, bits [15:10] of
the evtCount field behave as RES0.

While the actual PMU emulation code gets this right (i.e. RES0 bits are
masked out when programming the perf event), the sysreg emulation writes
an unmasked value to the in-memory cpu context. The net effect is that
guest reads and writes of PMEVTYPER<n>_EL0 will see non-RES0 behavior in
the reserved bits of the field.

As it so happens, kvm_pmu_set_counter_event_type() already writes a
masked value to the in-memory context that gets overwritten by
access_pmu_evtyper(). Fix the issue by removing the unnecessary (and
incorrect) register write in access_pmu_evtyper().
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Link: https://lore.kernel.org/r/20230713221649.3889210-1-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/sys_regs.c

@@ -986,7 +986,6 @@ static bool access_pmu_evtyper(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
 
 	if (p->is_write) {
 		kvm_pmu_set_counter_event_type(vcpu, p->regval, idx);
-		__vcpu_sys_reg(vcpu, reg) = p->regval & ARMV8_PMU_EVTYPE_MASK;
 		kvm_vcpu_pmu_restore_guest(vcpu);
 	} else {
 		p->regval = __vcpu_sys_reg(vcpu, reg) & ARMV8_PMU_EVTYPE_MASK;