[PATCH] KVM: MMU: make PTE_PREFETCH_NUM tunable

[Sergey Senozhatsky <senozhatsky@chromium.org>]

Turn PTE_PREFETCH_NUM into a module parameter, so that it
can be tuned per-VM.

- /sys/module/kvm/parameters/pte_prefetch_num 8

             VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time         Avg time

       EPT_VIOLATION     760998    54.85%     7.23%      0.92us  31765.89us      7.78us ( +-   1.46% )
           MSR_WRITE     170599    12.30%     0.53%      0.60us   3334.13us      2.52us ( +-   0.86% )
  EXTERNAL_INTERRUPT     159510    11.50%     1.65%      0.49us  43705.81us      8.45us ( +-   7.54% )
[..]

Total Samples:1387305, Total events handled time:81900258.99us.

- /sys/module/kvm/parameters/pte_prefetch_num 16

             VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time         Avg time

       EPT_VIOLATION     658064    52.58%     7.04%      0.91us  17022.84us      8.34us ( +-   1.52% )
           MSR_WRITE     163776    13.09%     0.54%      0.56us   5192.10us      2.57us ( +-   1.25% )
  EXTERNAL_INTERRUPT     144588    11.55%     1.62%      0.48us  97410.16us      8.75us ( +-  11.44% )
[..]

Total Samples:1251546, Total events handled time:77956187.56us.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 arch/x86/kvm/mmu/mmu.c | 31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 24a9f4c3f5e7..0ab4490674ec 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -115,6 +115,8 @@ module_param(dbg, bool, 0644);
 #endif
 
 #define PTE_PREFETCH_NUM		8
+static uint __read_mostly pte_prefetch_num = PTE_PREFETCH_NUM;
+module_param(pte_prefetch_num, uint, 0644);
 
 #define PT32_LEVEL_BITS 10
 
@@ -732,7 +734,7 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
 
 	/* 1 rmap, 1 parent PTE per level, and the prefetched rmaps. */
 	r = kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_pte_list_desc_cache,
-				       1 + PT64_ROOT_MAX_LEVEL + PTE_PREFETCH_NUM);
+				       1 + PT64_ROOT_MAX_LEVEL + pte_prefetch_num);
 	if (r)
 		return r;
 	r = kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_shadow_page_cache,
@@ -2753,20 +2755,29 @@ static int direct_pte_prefetch_many(struct kvm_vcpu *vcpu,
 				    struct kvm_mmu_page *sp,
 				    u64 *start, u64 *end)
 {
-	struct page *pages[PTE_PREFETCH_NUM];
+	struct page **pages;
 	struct kvm_memory_slot *slot;
 	unsigned int access = sp->role.access;
 	int i, ret;
 	gfn_t gfn;
 
+	pages = kmalloc_array(pte_prefetch_num, sizeof(struct page *),
+			      GFP_KERNEL);
+	if (!pages)
+		return -1;
+
 	gfn = kvm_mmu_page_get_gfn(sp, start - sp->spt);
 	slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn, access & ACC_WRITE_MASK);
-	if (!slot)
-		return -1;
+	if (!slot) {
+		ret = -1;
+		goto out;
+	}
 
 	ret = gfn_to_page_many_atomic(slot, gfn, pages, end - start);
-	if (ret <= 0)
-		return -1;
+	if (ret <= 0) {
+		ret = -1;
+		goto out;
+	}
 
 	for (i = 0; i < ret; i++, gfn++, start++) {
 		mmu_set_spte(vcpu, slot, start, access, gfn,
@@ -2774,7 +2785,9 @@ static int direct_pte_prefetch_many(struct kvm_vcpu *vcpu,
 		put_page(pages[i]);
 	}
 
-	return 0;
+out:
+	kfree(pages);
+	return ret;
 }
 
 static void __direct_pte_prefetch(struct kvm_vcpu *vcpu,
@@ -2785,10 +2798,10 @@ static void __direct_pte_prefetch(struct kvm_vcpu *vcpu,
 
 	WARN_ON(!sp->role.direct);
 
-	i = (sptep - sp->spt) & ~(PTE_PREFETCH_NUM - 1);
+	i = (sptep - sp->spt) & ~(pte_prefetch_num - 1);
 	spte = sp->spt + i;
 
-	for (i = 0; i < PTE_PREFETCH_NUM; i++, spte++) {
+	for (i = 0; i < pte_prefetch_num; i++, spte++) {
 		if (is_shadow_present_pte(*spte) || spte == sptep) {
 			if (!start)
 				continue;
-- 
2.33.0.882.g93a45727a2-goog