KVM: arm64: Avoid BBM when changing only s/w bits in Stage-2 PTE

Break-before-make (BBM) can be expensive, as transitioning via an
invalid mapping (i.e. the "break" step) requires the completion of TLB
invalidation and can also cause other agents to fault concurrently on
the invalid mapping.

Since BBM is not required when changing only the software bits of a PTE,
avoid the sequence in this case and just update the PTE directly.
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240423150538.2103045-9-tabba@google.comSigned-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/hyp/pgtable.c

@@ -972,6 +972,21 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx,
 	if (!stage2_pte_needs_update(ctx->old, new))
 		return -EAGAIN;
 
+	/* If we're only changing software bits, then store them and go! */
+	if (!kvm_pgtable_walk_shared(ctx) &&
+	    !((ctx->old ^ new) & ~KVM_PTE_LEAF_ATTR_HI_SW)) {
+		bool old_is_counted = stage2_pte_is_counted(ctx->old);
+
+		if (old_is_counted != stage2_pte_is_counted(new)) {
+			if (old_is_counted)
+				mm_ops->put_page(ctx->ptep);
+			else
+				mm_ops->get_page(ctx->ptep);
+		}
+		WARN_ON_ONCE(!stage2_try_set_pte(ctx, new));
+		return 0;
+	}
+
 	if (!stage2_try_break_pte(ctx, data->mmu))
 		return -EAGAIN;