KVM: arm64: Fix S1/S2 combination when FWB==1 and S2 has Device memory type

The G.a revision of the ARM ARM had it pretty clear that HCR_EL2.FWB
had no influence on "The way that stage 1 memory types and attributes
are combined with stage 2 Device type and attributes." (D5.5.5).

However, this wording was lost in further revisions of the architecture.

Restore the intended behaviour, which is to take the strongest memory
type of S1 and S2 in this case, as if FWB was 0. The specification is
being fixed accordingly.

Fixes: be04ceb ("KVM: arm64: nv: Add emulation of AT S12E{0,1}{R,W}")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20241125094756.609590-1-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: Marc Zyngier

arch/arm64/kvm/at.c

@@ -739,8 +739,15 @@ static u64 compute_par_s12(struct kvm_vcpu *vcpu, u64 s1_par,
 			final_attr = s1_parattr;
 			break;
 		default:
-			/* MemAttr[2]=0, Device from S2 */
-			final_attr = s2_memattr & GENMASK(1,0) << 2;
+			/*
+			 * MemAttr[2]=0, Device from S2.
+			 *
+			 * FWB does not influence the way that stage 1
+			 * memory types and attributes are combined
+			 * with stage 2 Device type and attributes.
+			 */
+			final_attr = min(s2_memattr_to_attr(s2_memattr),
+					 s1_parattr);
 		}
 	} else {
 		/* Combination of R_HMNDG, R_TNHFM and R_GQFSF */