KVM: x86/mmu: Drop the intermediate "transient" __kvm_sync_page()

Nove the kvm_unlink_unsync_page() call out of kvm_sync_page() and into
it's sole caller, and fold __kvm_sync_page() into kvm_sync_page() since
the latter becomes a pure pass-through.  There really should be no reason
for code to do a complete sync of a shadow page outside of the full
kvm_mmu_sync_roots(), e.g. the one use case that creeped in turned out to
be flawed and counter-productive.

Drop the stale comment about @sp->gfn needing to be write-protected, as
it directly contradicts the kvm_mmu_get_page() usage.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-13-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -1780,9 +1780,8 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
 	  &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)])	\
 		if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else
 
-/* @sp->gfn should be write-protected at the call site */
-static bool __kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
-			    struct list_head *invalid_list)
+static bool kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
+			 struct list_head *invalid_list)
 {
 	if (vcpu->arch.mmu->sync_page(vcpu, sp) == 0) {
 		kvm_mmu_prepare_zap_page(vcpu->kvm, sp, invalid_list);
@@ -1830,13 +1829,6 @@ static bool is_obsolete_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
 	       unlikely(sp->mmu_valid_gen != kvm->arch.mmu_valid_gen);
 }
 
-static bool kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
-			 struct list_head *invalid_list)
-{
-	kvm_unlink_unsync_page(vcpu->kvm, sp);
-	return __kvm_sync_page(vcpu, sp, invalid_list);
-}
-
 struct mmu_page_path {
 	struct kvm_mmu_page *parent[PT64_ROOT_MAX_LEVEL];
 	unsigned int idx[PT64_ROOT_MAX_LEVEL];
@@ -1931,6 +1923,7 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
 		}
 
 		for_each_sp(pages, sp, parents, i) {
+			kvm_unlink_unsync_page(vcpu->kvm, sp);
 			flush |= kvm_sync_page(vcpu, sp, &invalid_list);
 			mmu_pages_clear_parents(&parents);
 		}
@@ -2009,7 +2002,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 
 		if (sp->unsync) {
 			/*
-			 * The page is good, but is stale.  __kvm_sync_page does
+			 * The page is good, but is stale.  kvm_sync_page does
 			 * get the latest guest state, but (unlike mmu_unsync_children)
 			 * it doesn't write-protect the page or mark it synchronized!
 			 * This way the validity of the mapping is ensured, but the
@@ -2020,7 +2013,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 			 * If the sync fails, the page is zapped.  If so, break
 			 * in order to rebuild it.
 			 */
-			if (!__kvm_sync_page(vcpu, sp, &invalid_list))
+			if (!kvm_sync_page(vcpu, sp, &invalid_list))
 				break;
 
 			WARN_ON(!list_empty(&invalid_list));