Revert "KVM: MMU: drop kvm_mmu_zap_mmio_sptes"

Revert back to a dedicated (and slower) mechanism for handling the
scenario where all MMIO shadow PTEs need to be zapped due to overflowing
the MMIO generation number.  The MMIO generation scenario is almost
literally a one-in-a-million occurrence, i.e. is not a performance
sensitive scenario.

Restoring kvm_mmu_zap_mmio_sptes() leaves VM teardown as the only user
of kvm_mmu_invalidate_zap_all_pages() and paves the way for removing
the fast invalidate mechanism altogether.

This reverts commit a8eca9dc.

Cc: Xiao Guangrong <guangrong.xiao@gmail.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -319,6 +319,7 @@ struct kvm_mmu_page {
 	struct list_head link;
 	struct hlist_node hash_link;
 	bool unsync;
+	bool mmio_cached;
 
 	/*
 	 * The following two entries are used to key the shadow page in the

arch/x86/kvm/mmu.c

@@ -391,6 +391,8 @@ static void mark_mmio_spte(struct kvm_vcpu *vcpu, u64 *sptep, u64 gfn,
 	mask |= (gpa & shadow_nonpresent_or_rsvd_mask)
 		<< shadow_nonpresent_or_rsvd_mask_len;
 
+	page_header(__pa(sptep))->mmio_cached = true;
+
 	trace_mark_mmio_spte(sptep, gfn, access, gen);
 	mmu_spte_set(sptep, mask);
 }
@@ -5942,6 +5944,24 @@ static bool kvm_has_zapped_obsolete_pages(struct kvm *kvm)
 	return unlikely(!list_empty_careful(&kvm->arch.zapped_obsolete_pages));
 }
 
+static void kvm_mmu_zap_mmio_sptes(struct kvm *kvm)
+{
+	struct kvm_mmu_page *sp, *node;
+	LIST_HEAD(invalid_list);
+
+	spin_lock(&kvm->mmu_lock);
+restart:
+	list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
+		if (!sp->mmio_cached)
+			continue;
+		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
+			goto restart;
+	}
+
+	kvm_mmu_commit_zap_page(kvm, &invalid_list);
+	spin_unlock(&kvm->mmu_lock);
+}
+
 void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, u64 gen)
 {
 	WARN_ON(gen & KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS);
@@ -5963,7 +5983,7 @@ void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, u64 gen)
 	 */
 	if (unlikely(gen == 0)) {
 		kvm_debug_ratelimited("kvm: zapping shadow pages for mmio generation wraparound\n");
-		kvm_mmu_invalidate_zap_all_pages(kvm);
+		kvm_mmu_zap_mmio_sptes(kvm);
 	}
 }