[PATCH v3 2/2] KVM: x86/mmu: Split huge pages mapped by the TDP MMU on fault

[David Matlack <dmatlack@google.com>]

Now that the TDP MMU has a mechanism to split huge pages, use it in the
fault path when a huge page needs to be replaced with a mapping at a
lower level.

This change reduces the negative performance impact of NX HugePages.
Prior to this change if a vCPU executed from a huge page and NX
HugePages was enabled, the vCPU would take a fault, zap the huge page,
and mapping the faulting address at 4KiB with execute permissions
enabled. The rest of the memory would be left *unmapped* and have to be
faulted back in by the guest upon access (read, write, or execute). If
guest is backed by 1GiB, a single execute instruction can zap an entire
GiB of its physical address space.

For example, it can take a VM longer to execute from its memory than to
populate that memory in the first place:

$ ./execute_perf_test -s anonymous_hugetlb_1gb -v96

Populating memory             : 2.748378795s
Executing from memory         : 2.899670885s

With this change, such faults split the huge page instead of zapping it,
which avoids the non-present faults on the rest of the huge page:

$ ./execute_perf_test -s anonymous_hugetlb_1gb -v96

Populating memory             : 2.729544474s
Executing from memory         : 0.111965688s   <---

This change also reduces the performance impact of dirty logging when
eager_page_split=N. eager_page_split=N (abbreviated "eps=N" below) can
be desirable for read-heavy workloads, as it avoids allocating memory to
split huge pages that are never written and avoids increasing the TLB
miss cost on reads of those pages.

             | Config: ept=Y, tdp_mmu=Y, 5% writes           |
             | Iteration 1 dirty memory time                 |
             | --------------------------------------------- |
vCPU Count   | eps=N (Before) | eps=N (After) | eps=Y        |
------------ | -------------- | ------------- | ------------ |
2            | 0.332305091s   | 0.019615027s  | 0.006108211s |
4            | 0.353096020s   | 0.019452131s  | 0.006214670s |
8            | 0.453938562s   | 0.019748246s  | 0.006610997s |
16           | 0.719095024s   | 0.019972171s  | 0.007757889s |
32           | 1.698727124s   | 0.021361615s  | 0.012274432s |
64           | 2.630673582s   | 0.031122014s  | 0.016994683s |
96           | 3.016535213s   | 0.062608739s  | 0.044760838s |

Eager page splitting remains beneficial for write-heavy workloads, but
the gap is now reduced.

             | Config: ept=Y, tdp_mmu=Y, 100% writes         |
             | Iteration 1 dirty memory time                 |
             | --------------------------------------------- |
vCPU Count   | eps=N (Before) | eps=N (After) | eps=Y        |
------------ | -------------- | ------------- | ------------ |
2            | 0.317710329s   | 0.296204596s  | 0.058689782s |
4            | 0.337102375s   | 0.299841017s  | 0.060343076s |
8            | 0.386025681s   | 0.297274460s  | 0.060399702s |
16           | 0.791462524s   | 0.298942578s  | 0.062508699s |
32           | 1.719646014s   | 0.313101996s  | 0.075984855s |
64           | 2.527973150s   | 0.455779206s  | 0.079789363s |
96           | 2.681123208s   | 0.673778787s  | 0.165386739s |

Further study is needed to determine if the remaining gap is acceptable
for customer workloads or if eager_page_split=N still requires a-priori
knowledge of the VM workload, especially when considering these costs
extrapolated out to large VMs with e.g. 416 vCPUs and 12TB RAM.

Signed-off-by: David Matlack <dmatlack@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 73 ++++++++++++++++++--------------------
 1 file changed, 35 insertions(+), 38 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 4e5b3ae824c1..e08596775427 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1146,6 +1146,9 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
 	return 0;
 }
 
+static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
+				   struct kvm_mmu_page *sp, bool shared);
+
 /*
  * Handle a TDP page fault (NPT/EPT violation/misconfiguration) by installing
  * page tables and SPTEs to translate the faulting guest physical address.
@@ -1171,49 +1174,42 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		if (iter.level == fault->goal_level)
 			break;
 
-		/*
-		 * If there is an SPTE mapping a large page at a higher level
-		 * than the target, that SPTE must be cleared and replaced
-		 * with a non-leaf SPTE.
-		 */
+		/* Step down into the lower level page table if it exists. */
 		if (is_shadow_present_pte(iter.old_spte) &&
-		    is_large_pte(iter.old_spte)) {
-			if (tdp_mmu_zap_spte_atomic(vcpu->kvm, &iter))
-				break;
+		    !is_large_pte(iter.old_spte))
+			continue;
 
-			/*
-			 * The iter must explicitly re-read the spte here
-			 * because the new value informs the !present
-			 * path below.
-			 */
-			iter.old_spte = kvm_tdp_mmu_read_spte(iter.sptep);
-		}
+		/*
+		 * If SPTE has been frozen by another thread, just give up and
+		 * retry, avoiding unnecessary page table allocation and free.
+		 */
+		if (is_removed_spte(iter.old_spte))
+			break;
 
-		if (!is_shadow_present_pte(iter.old_spte)) {
-			/*
-			 * If SPTE has been frozen by another thread, just
-			 * give up and retry, avoiding unnecessary page table
-			 * allocation and free.
-			 */
-			if (is_removed_spte(iter.old_spte))
-				break;
+		/*
+		 * The SPTE is either non-present or points to a huge page that
+		 * needs to be split.
+		 */
+		sp = tdp_mmu_alloc_sp(vcpu);
+		tdp_mmu_init_child_sp(sp, &iter);
 
-			sp = tdp_mmu_alloc_sp(vcpu);
-			tdp_mmu_init_child_sp(sp, &iter);
+		sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
 
-			sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
+		if (is_shadow_present_pte(iter.old_spte))
+			ret = tdp_mmu_split_huge_page(kvm, &iter, sp, true);
+		else
+			ret = tdp_mmu_link_sp(kvm, &iter, sp, true);
 
-			if (tdp_mmu_link_sp(kvm, &iter, sp, true)) {
-				tdp_mmu_free_sp(sp);
-				break;
-			}
+		if (ret) {
+			tdp_mmu_free_sp(sp);
+			break;
+		}
 
-			if (fault->huge_page_disallowed &&
-			    fault->req_level >= iter.level) {
-				spin_lock(&kvm->arch.tdp_mmu_pages_lock);
-				track_possible_nx_huge_page(kvm, sp);
-				spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
-			}
+		if (fault->huge_page_disallowed &&
+		    fault->req_level >= iter.level) {
+			spin_lock(&kvm->arch.tdp_mmu_pages_lock);
+			track_possible_nx_huge_page(kvm, sp);
+			spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
 		}
 	}
 
@@ -1477,6 +1473,7 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
 	return sp;
 }
 
+/* Note, the caller is responsible for initializing @sp. */
 static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
 				   struct kvm_mmu_page *sp, bool shared)
 {
@@ -1484,8 +1481,6 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
 	const int level = iter->level;
 	int ret, i;
 
-	tdp_mmu_init_child_sp(sp, iter);
-
 	/*
 	 * No need for atomics when writing to sp->spt since the page table has
 	 * not been linked in yet and thus is not reachable from any other CPU.
@@ -1561,6 +1556,8 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
 				continue;
 		}
 
+		tdp_mmu_init_child_sp(sp, &iter);
+
 		if (tdp_mmu_split_huge_page(kvm, &iter, sp, shared))
 			goto retry;
 
-- 
2.38.1.431.g37b22c650d-goog