KVM: arm64: Allow userspace to change ID_AA64PFR0_EL1

Allow userspace to change the guest-visible value of the register with
some severe limitation:

 - No changes to features not virtualized by KVM (AMU, MPAM, RAS)

 - Short of full GICv2 emulation in kernel, hiding GICv3 from the guest
   makes absolutely no sense.

 - FP is effectively assumed for KVM VMs.

Signed-off-by: Jing Zhang <jingzhangos@google.com>
[oliver: restrict features that are illogical to change]
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231003230408.3405722-9-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: jingzhangos

arch/arm64/kvm/sys_regs.c

@@ -2057,7 +2057,13 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	  .get_user = get_id_reg,
 	  .set_user = set_id_reg,
 	  .reset = read_sanitised_id_aa64pfr0_el1,
-	  .val = ID_AA64PFR0_EL1_CSV2_MASK | ID_AA64PFR0_EL1_CSV3_MASK, },
+	  .val = ~(ID_AA64PFR0_EL1_AMU |
+		   ID_AA64PFR0_EL1_MPAM |
+		   ID_AA64PFR0_EL1_SVE |
+		   ID_AA64PFR0_EL1_RAS |
+		   ID_AA64PFR0_EL1_GIC |
+		   ID_AA64PFR0_EL1_AdvSIMD |
+		   ID_AA64PFR0_EL1_FP), },
 	ID_SANITISED(ID_AA64PFR1_EL1),
 	ID_UNALLOCATED(4,2),
 	ID_UNALLOCATED(4,3),