KVM: arm64: Add filtering hook to S1 page table walk

Add a filtering hook that can get called on each level of the
walk, and providing access to the full state.

Crucially, this is called *before* the access is made, so that
it is possible to track down the level of a faulting access.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Marc Zyngier

arch/arm64/include/asm/kvm_nested.h

@@ -288,7 +288,21 @@ enum trans_regime {
 	TR_EL2,
 };
 
+struct s1_walk_info;
+
+struct s1_walk_context {
+	struct s1_walk_info	*wi;
+	u64			table_ipa;
+	int			level;
+};
+
+struct s1_walk_filter {
+	int	(*fn)(struct s1_walk_context *, void *);
+	void	*priv;
+};
+
 struct s1_walk_info {
+	struct s1_walk_filter	*filter;
 	u64	     		baddr;
 	enum trans_regime	regime;
 	unsigned int		max_oa_bits;

arch/arm64/kvm/at.c

@@ -404,6 +404,17 @@ static int walk_s1(struct kvm_vcpu *vcpu, struct s1_walk_info *wi,
 			ipa = kvm_s2_trans_output(&s2_trans);
 		}
 
+		if (wi->filter) {
+			ret = wi->filter->fn(&(struct s1_walk_context)
+					     {
+						     .wi	= wi,
+						     .table_ipa	= baddr,
+						     .level	= level,
+					     }, wi->filter->priv);
+			if (ret)
+				return ret;
+		}
+
 		ret = kvm_read_guest(vcpu->kvm, ipa, &desc, sizeof(desc));
 		if (ret) {
 			fail_s1_walk(wr, ESR_ELx_FSC_SEA_TTW(level), false);