KVM: arm64: nv: Fix ATS12 handling of single-stage translation

Volodymyr reports that using a Xen DomU as a nested guest (where
HCR_EL2.E2H == 0), ATS12 results in a translation that stops at
the L2's S1, which isn't something you'd normally expects.

Comparing the code against the spec proves to be illuminating,
and suggests that the author of such code must have been tired,
cross-eyed, drunk, or maybe all of the above.

The gist of it is that, apart from HCR_EL2.VM or HCR_EL2.DC being
0, only the use of the EL2&0 translation regime limits the walk
to S1 only, and that we must finish the S2 walk in any other case.
Which solves the above issue, as E2H==0 indicates that ATS12 walks
the EL1&0 translation regime.

Explicitly checking for EL2&0 fixes this.

Reported-by: Volodymyr Babchuk <[email protected]>
Suggested-by: Oliver Upton <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Fixes: be04ceb ("KVM: arm64: nv: Add emulation of AT S12E{0,1}{R,W}")
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Oliver Upton <[email protected]>

Author: Marc Zyngier

arch/arm64/kvm/at.c

@@ -1420,10 +1420,10 @@ void __kvm_at_s12(struct kvm_vcpu *vcpu, u32 op, u64 vaddr)
 		return;
 
 	/*
-	 * If we only have a single stage of translation (E2H=0 or
-	 * TGE=1), exit early. Same thing if {VM,DC}=={0,0}.
+	 * If we only have a single stage of translation (EL2&0), exit
+	 * early. Same thing if {VM,DC}=={0,0}.
 	 */
-	if (!vcpu_el2_e2h_is_set(vcpu) || vcpu_el2_tge_is_set(vcpu) ||
+	if (compute_translation_regime(vcpu, op) == TR_EL20 ||
 	    !(vcpu_read_sys_reg(vcpu, HCR_EL2) & (HCR_VM | HCR_DC)))
 		return;