[PATCH v4 11/12] KVM: x86/mmu: split a single gfn zap range when guest MTRRs are honored

[Yan Zhao]

Split a single gfn zap range (specifially range [0, ~0UL)) to smaller
ranges according to current memslot layout when guest MTRRs are honored.

Though vCPUs have been serialized to perform kvm_zap_gfn_range() for MTRRs
updates and CR0.CD toggles, contention caused rescheduling cost is still
huge when there're concurrent page fault holding mmu_lock for read.

Split a single huge zap range according to the actual memslot layout can
reduce unnecessary transversal and yielding cost in tdp mmu.
Also, it can increase the chances for larger ranges to find existing ranges
to zap in zap list.

Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
---
 arch/x86/kvm/mtrr.c | 39 +++++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
index 9fdbdbf874a8..00e98dfc4b0d 100644
--- a/arch/x86/kvm/mtrr.c
+++ b/arch/x86/kvm/mtrr.c
@@ -909,21 +909,44 @@ static void kvm_zap_or_wait_mtrr_zap_list(struct kvm *kvm)
 static void kvm_mtrr_zap_gfn_range(struct kvm_vcpu *vcpu,
 				   gfn_t gfn_start, gfn_t gfn_end)
 {
+	int idx = srcu_read_lock(&vcpu->kvm->srcu);
+	const struct kvm_memory_slot *memslot;
 	struct mtrr_zap_range *range;
+	struct kvm_memslot_iter iter;
+	struct kvm_memslots *slots;
+	gfn_t start, end;
+	int i;
 
-	range = kmalloc(sizeof(*range), GFP_KERNEL_ACCOUNT);
-	if (!range)
-		goto fail;
-
-	range->start = gfn_start;
-	range->end = gfn_end;
-
-	kvm_add_mtrr_zap_list(vcpu->kvm, range);
+	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
+		slots = __kvm_memslots(vcpu->kvm, i);
+		kvm_for_each_memslot_in_gfn_range(&iter, slots, gfn_start, gfn_end) {
+			memslot = iter.slot;
+			start = max(gfn_start, memslot->base_gfn);
+			end = min(gfn_end, memslot->base_gfn + memslot->npages);
+			if (WARN_ON_ONCE(start >= end))
+				continue;
+
+			range = kmalloc(sizeof(*range), GFP_KERNEL_ACCOUNT);
+			if (!range)
+				goto fail;
+
+			range->start = start;
+			range->end = end;
+
+			/*
+			 * Redundent ranges in different address space will be
+			 * removed in kvm_add_mtrr_zap_list().
+			 */
+			kvm_add_mtrr_zap_list(vcpu->kvm, range);
+		}
+	}
+	srcu_read_unlock(&vcpu->kvm->srcu, idx);
 
 	kvm_zap_or_wait_mtrr_zap_list(vcpu->kvm);
 	return;
 
 fail:
+	srcu_read_unlock(&vcpu->kvm->srcu, idx);
 	kvm_zap_gfn_range(vcpu->kvm, gfn_start, gfn_end);
 }
 
-- 
2.17.1