KVM: arm64: Allow control of dpISA extensions in ID_AA64ISAR3_EL1

ID_AA64ISAR3_EL1 is currently marked as unallocated in KVM but does have a
number of bitfields defined in it. Expose FPRCVT and FAMINMAX, two simple
instruction only extensions to guests.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250107-arm64-2024-dpisa-v5-4-7578da51fc3d@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

Author: broonie

arch/arm64/kvm/sys_regs.c

@@ -1604,6 +1604,9 @@ static u64 __kvm_read_sanitised_id_reg(const struct kvm_vcpu *vcpu,
 		if (!cpus_have_final_cap(ARM64_HAS_WFXT))
 			val &= ~ARM64_FEATURE_MASK(ID_AA64ISAR2_EL1_WFxT);
 		break;
+	case SYS_ID_AA64ISAR3_EL1:
+		val &= ID_AA64ISAR3_EL1_FPRCVT | ID_AA64ISAR3_EL1_FAMINMAX;
+		break;
 	case SYS_ID_AA64MMFR2_EL1:
 		val &= ~ID_AA64MMFR2_EL1_CCIDX_MASK;
 		break;
@@ -2608,7 +2611,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	ID_WRITABLE(ID_AA64ISAR2_EL1, ~(ID_AA64ISAR2_EL1_RES0 |
 					ID_AA64ISAR2_EL1_APA3 |
 					ID_AA64ISAR2_EL1_GPA3)),
-	ID_UNALLOCATED(6,3),
+	ID_WRITABLE(ID_AA64ISAR3_EL1, (ID_AA64ISAR3_EL1_FPRCVT |
+				       ID_AA64ISAR3_EL1_FAMINMAX)),
 	ID_UNALLOCATED(6,4),
 	ID_UNALLOCATED(6,5),
 	ID_UNALLOCATED(6,6),