[PATCH 3/3] 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 for the same reasons as above but write faults.
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>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 37 +++++++++++++++++++++++++------------
 1 file changed, 25 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 9263765c8068..5a2120d85347 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1131,6 +1131,10 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
 	return 0;
 }
 
+static int tdp_mmu_split_huge_page_atomic(struct kvm_vcpu *vcpu,
+					  struct tdp_iter *iter,
+					  bool account_nx);
+
 /*
  * Handle a TDP page fault (NPT/EPT violation/misconfiguration) by installing
  * page tables and SPTEs to translate the faulting guest physical address.
@@ -1140,6 +1144,7 @@ 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;
+	bool account_nx;
 	int ret;
 
 	kvm_mmu_hugepage_adjust(vcpu, fault);
@@ -1155,28 +1160,22 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		if (iter.level == fault->goal_level)
 			break;
 
+		account_nx = fault->huge_page_disallowed &&
+			     fault->req_level >= iter.level;
+
 		/*
 		 * 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.
+		 * than the target, split it down one level.
 		 */
 		if (is_shadow_present_pte(iter.old_spte) &&
 		    is_large_pte(iter.old_spte)) {
-			if (tdp_mmu_zap_spte_atomic(vcpu->kvm, &iter))
+			if (tdp_mmu_split_huge_page_atomic(vcpu, &iter, account_nx))
 				break;
 
-			/*
-			 * 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);
+			continue;
 		}
 
 		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
@@ -1496,6 +1495,20 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
 	return ret;
 }
 
+static int tdp_mmu_split_huge_page_atomic(struct kvm_vcpu *vcpu,
+					  struct tdp_iter *iter,
+					  bool account_nx)
+{
+	struct kvm_mmu_page *sp = tdp_mmu_alloc_sp(vcpu);
+	int r;
+
+	r = tdp_mmu_split_huge_page(vcpu->kvm, iter, sp, true, account_nx);
+	if (r)
+		tdp_mmu_free_sp(sp);
+
+	return r;
+}
+
 static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
 					 struct kvm_mmu_page *root,
 					 gfn_t start, gfn_t end,
-- 
2.35.1.1094.g7c7d902a7c-goog