KVM: x86/mmu: Allow NULL @vcpu in kvm_mmu_find_shadow_page()

Allow @vcpu to be NULL in kvm_mmu_find_shadow_page() (and its only
caller __kvm_mmu_get_shadow_page()). @vcpu is only required to sync
indirect shadow pages, so it's safe to pass in NULL when looking up
direct shadow pages.

This will be used for doing eager page splitting, which allocates direct
shadow pages from the context of a VM ioctl without access to a vCPU
pointer.
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220516232138.1783324-14-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -1975,6 +1975,12 @@ static void clear_sp_write_flooding_count(u64 *spte)
 	__clear_sp_write_flooding_count(sptep_to_sp(spte));
 }
 
+/*
+ * The vCPU is required when finding indirect shadow pages; the shadow
+ * page may already exist and syncing it needs the vCPU pointer in
+ * order to read guest page tables.  Direct shadow pages are never
+ * unsync, thus @vcpu can be NULL if @role.direct is true.
+ */
 static struct kvm_mmu_page *kvm_mmu_find_shadow_page(struct kvm *kvm,
 						     struct kvm_vcpu *vcpu,
 						     gfn_t gfn,
@@ -2013,6 +2019,9 @@ static struct kvm_mmu_page *kvm_mmu_find_shadow_page(struct kvm *kvm,
 			goto out;
 
 		if (sp->unsync) {
+			if (KVM_BUG_ON(!vcpu, kvm))
+				break;
+
 			/*
 			 * The page is good, but is stale.  kvm_sync_page does
 			 * get the latest guest state, but (unlike mmu_unsync_children)
@@ -2090,6 +2099,7 @@ static struct kvm_mmu_page *kvm_mmu_alloc_shadow_page(struct kvm *kvm,
 	return sp;
 }
 
+/* Note, @vcpu may be NULL if @role.direct is true; see kvm_mmu_find_shadow_page. */
 static struct kvm_mmu_page *__kvm_mmu_get_shadow_page(struct kvm *kvm,
 						      struct kvm_vcpu *vcpu,
 						      struct shadow_page_caches *caches,