KVM: x86/mmu: Drop per-VM zapped_obsolete_pages list

Drop the per-VM zapped_obsolete_pages list now that the usage from the
defunct mmu_shrinker is gone, and instead use a local list to track pages
in kvm_zap_obsolete_pages(), the sole remaining user of
zapped_obsolete_pages.

Opportunistically add an assertion to verify and document that slots_lock
must be held, i.e. that there can only be one active instance of
kvm_zap_obsolete_pages() at any given time, and by doing so also prove
that using a local list instead of a per-VM list doesn't change any
functionality (beyond trivialities like list initialization).

Signed-off-by: Vipin Sharma <vipinsh@google.com>
Link: https://lore.kernel.org/r/20241101201437.1604321-2-vipinsh@google.com
[sean: split to separate patch, write changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: shvipin

arch/x86/include/asm/kvm_host.h

@@ -1306,7 +1306,6 @@ struct kvm_arch {
 	bool pre_fault_allowed;
 	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
 	struct list_head active_mmu_pages;
-	struct list_head zapped_obsolete_pages;
 	/*
 	 * A list of kvm_mmu_page structs that, if zapped, could possibly be
 	 * replaced by an NX huge page.  A shadow page is on this list if its

arch/x86/kvm/mmu/mmu.c

@@ -6367,8 +6367,11 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
 {
 	struct kvm_mmu_page *sp, *node;
 	int nr_zapped, batch = 0;
+	LIST_HEAD(invalid_list);
 	bool unstable;
 
+	lockdep_assert_held(&kvm->slots_lock);
+
 restart:
 	list_for_each_entry_safe_reverse(sp, node,
 	      &kvm->arch.active_mmu_pages, link) {
@@ -6400,7 +6403,7 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
 		}
 
 		unstable = __kvm_mmu_prepare_zap_page(kvm, sp,
-				&kvm->arch.zapped_obsolete_pages, &nr_zapped);
+				&invalid_list, &nr_zapped);
 		batch += nr_zapped;
 
 		if (unstable)
@@ -6416,7 +6419,7 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
 	 * kvm_mmu_load()), and the reload in the caller ensure no vCPUs are
 	 * running with an obsolete MMU.
 	 */
-	kvm_mmu_commit_zap_page(kvm, &kvm->arch.zapped_obsolete_pages);
+	kvm_mmu_commit_zap_page(kvm, &invalid_list);
 }
 
 /*
@@ -6483,7 +6486,6 @@ void kvm_mmu_init_vm(struct kvm *kvm)
 {
 	kvm->arch.shadow_mmio_value = shadow_mmio_value;
 	INIT_LIST_HEAD(&kvm->arch.active_mmu_pages);
-	INIT_LIST_HEAD(&kvm->arch.zapped_obsolete_pages);
 	INIT_LIST_HEAD(&kvm->arch.possible_nx_huge_pages);
 	spin_lock_init(&kvm->arch.mmu_unsync_pages_lock);