KVM: arm64: Don't defer TLB invalidation when zapping table entries

Commit 7657ea92 ("KVM: arm64: Use TLBI range-based instructions for
unmap") introduced deferred TLB invalidation for the stage-2 page-table
so that range-based invalidation can be used for the accumulated
addresses. This works fine if the structure of the page-tables remains
unchanged, but if entire tables are zapped and subsequently freed then
we transiently leave the hardware page-table walker with a reference
to freed memory thanks to the translation walk caches. For example,
stage2_unmap_walker() will free page-table pages:

	if (childp)
		mm_ops->put_page(childp);

and issue the TLB invalidation later in kvm_pgtable_stage2_unmap():

	if (stage2_unmap_defer_tlb_flush(pgt))
		/* Perform the deferred TLB invalidations */
		kvm_tlb_flush_vmid_range(pgt->mmu, addr, size);

For now, take the conservative approach and invalidate the TLB eagerly
when we clear a table entry. Note, however, that the existing level
hint passed to __kvm_tlb_flush_vmid_ipa() is incorrect and will be
fixed in a subsequent patch.

Cc: Raghavendra Rao Ananta <rananta@google.com>
Cc: Shaoqin Huang <shahuang@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240327124853.11206-2-will@kernel.orgSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/hyp/pgtable.c

@@ -896,10 +896,12 @@ static void stage2_unmap_put_pte(const struct kvm_pgtable_visit_ctx *ctx,
 	if (kvm_pte_valid(ctx->old)) {
 		kvm_clear_pte(ctx->ptep);
 
-		if (!stage2_unmap_defer_tlb_flush(pgt))
+		if (!stage2_unmap_defer_tlb_flush(pgt) ||
+		    kvm_pte_table(ctx->old, ctx->level)) {
 			kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, mmu,
 					ctx->addr, ctx->level);
 		}
+	}
 
 	mm_ops->put_page(ctx->ptep);
 }