[RFC PATCH 04/15] KVM: x86/mmu: Factor out logic to atomically install a new page table

[David Matlack <dmatlack@google.com>]

Factor out the logic to atomically replace an SPTE with an SPTE that
points to a new page table. This will be used in a follow-up commit to
split a large page SPTE into one level lower.

Signed-off-by: David Matlack <dmatlack@google.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 53 ++++++++++++++++++++++++++------------
 1 file changed, 37 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index cc9fe33c9b36..9ee3f4f7fdf5 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -945,6 +945,39 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
 	return ret;
 }
 
+/*
+ * tdp_mmu_install_sp_atomic - Atomically replace the given spte with an
+ * spte pointing to the provided page table.
+ *
+ * @kvm: kvm instance
+ * @iter: a tdp_iter instance currently on the SPTE that should be set
+ * @sp: The new TDP page table to install.
+ * @account_nx: True if this page table is being installed to split a
+ *              non-executable huge page.
+ *
+ * Returns: True if the new page table was installed. False if spte being
+ *          replaced changed, causing the atomic compare-exchange to fail.
+ *          If this function returns false the sp will be freed before
+ *          returning.
+ */
+static bool tdp_mmu_install_sp_atomic(struct kvm *kvm,
+				      struct tdp_iter *iter,
+				      struct kvm_mmu_page *sp,
+				      bool account_nx)
+{
+	u64 spte;
+
+	spte = make_nonleaf_spte(sp->spt, !shadow_accessed_mask);
+
+	if (tdp_mmu_set_spte_atomic(kvm, iter, spte)) {
+		tdp_mmu_link_page(kvm, sp, account_nx);
+		return true;
+	} else {
+		tdp_mmu_free_sp(sp);
+		return false;
+	}
+}
+
 /*
  * Handle a TDP page fault (NPT/EPT violation/misconfiguration) by installing
  * page tables and SPTEs to translate the faulting guest physical address.
@@ -954,8 +987,6 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 	struct kvm_mmu *mmu = vcpu->arch.mmu;
 	struct tdp_iter iter;
 	struct kvm_mmu_page *sp;
-	u64 *child_pt;
-	u64 new_spte;
 	int ret;
 
 	kvm_mmu_hugepage_adjust(vcpu, fault);
@@ -983,6 +1014,9 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		}
 
 		if (!is_shadow_present_pte(iter.old_spte)) {
+			bool account_nx = fault->huge_page_disallowed &&
+					  fault->req_level >= iter.level;
+
 			/*
 			 * If SPTE has been frozen by another thread, just
 			 * give up and retry, avoiding unnecessary page table
@@ -992,21 +1026,8 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 				break;
 
 			sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level - 1);
-			child_pt = sp->spt;
-
-			new_spte = make_nonleaf_spte(child_pt,
-						     !shadow_accessed_mask);
-
-			if (tdp_mmu_set_spte_atomic(vcpu->kvm, &iter, new_spte)) {
-				tdp_mmu_link_page(vcpu->kvm, sp,
-						  fault->huge_page_disallowed &&
-						  fault->req_level >= iter.level);
-
-				trace_kvm_mmu_get_page(sp, true);
-			} else {
-				tdp_mmu_free_sp(sp);
+			if (!tdp_mmu_install_sp_atomic(vcpu->kvm, &iter, sp, account_nx))
 				break;
-			}
 		}
 	}
 
-- 
2.34.0.rc2.393.gf8c9666880-goog