KVM: arm64: Don't advertise FEAT_SPE to guests

Even though KVM sets up MDCR_EL2 to trap accesses to the SPE buffer and
sampling control registers and to inject an undefined exception, the
presence of FEAT_SPE is still advertised in the ID_AA64DFR0_EL1 register,
if the hardware supports it. Getting an undefined exception when accessing
a register usually happens for a hardware feature which is not implemented,
and indeed this is how PMU emulation is handled when the virtual machine
has been created without the KVM_ARM_VCPU_PMU_V3 feature. Let's be
consistent and never advertise FEAT_SPE, because KVM doesn't have support
for emulating it yet.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210409152154.198566-3-alexandru.elisei@arm.com

arch/arm64/kvm/sys_regs.c

@@ -1063,6 +1063,8 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
 		val = cpuid_feature_cap_perfmon_field(val,
 						      ID_AA64DFR0_PMUVER_SHIFT,
 						      kvm_vcpu_has_pmu(vcpu) ? ID_AA64DFR0_PMUVER_8_4 : 0);
+		/* Hide SPE from guests */
+		val &= ~FEATURE(ID_AA64DFR0_PMSVER);
 		break;
 	case SYS_ID_DFR0_EL1:
 		/* Limit guests to PMUv3 for ARMv8.4 */