KVM: arm64: nv: Expose BTI and CSV_frac to a guest hypervisor

Now that we expose PAC to NV guests, we can also expose BTI (as
the two as joined at the hip, due to some of the PAC instructions
being landing pads).

While we're at it, also propagate CSV_frac, which requires no
particular emulation.

Fixes: f4f6a95b ("KVM: arm64: nv: Advertise support for PAuth")
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240528100632.1831995-3-maz@kernel.orgSigned-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/nested.c

@@ -58,8 +58,10 @@ static u64 limit_nv_id_reg(u32 id, u64 val)
 		break;
 
 	case SYS_ID_AA64PFR1_EL1:
-		/* Only support SSBS */
-		val &= NV_FTR(PFR1, SSBS);
+		/* Only support BTI, SSBS, CSV2_frac */
+		val &= (NV_FTR(PFR1, BT)	|
+			NV_FTR(PFR1, SSBS)	|
+			NV_FTR(PFR1, CSV2_frac));
 		break;
 
 	case SYS_ID_AA64MMFR0_EL1: