[PATCH v6 18/29] KVM: x86: Use nr_memslot_pages to avoid traversing the memslots array

["Maciej S. Szmigiero" <mail@xxxxxxxxxxxxxxxxxxxxx>]

From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>

There is no point in recalculating from scratch the total number of pages
in all memslots each time a memslot is created or deleted.  Use KVM's
cached nr_memslot_pages to compute the default max number of MMU pages.

Note that even with nr_memslot_pages capped at ULONG_MAX we can't safely
multiply it by KVM_PERMILLE_MMU_PAGES (20) since this operation can
possibly overflow an unsigned long variable.

Write this "* 20 / 1000" operation as "/ 50" instead to avoid such
overflow.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
[sean: use common KVM field and rework changelog accordingly]
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |  3 +--
 arch/x86/kvm/mmu/mmu.c          | 24 ------------------------
 arch/x86/kvm/x86.c              | 10 +++++++---
 3 files changed, 8 insertions(+), 29 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e41ad1ead721..a5daf399e033 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -135,7 +135,7 @@
 #define KVM_HPAGE_MASK(x)	(~(KVM_HPAGE_SIZE(x) - 1))
 #define KVM_PAGES_PER_HPAGE(x)	(KVM_HPAGE_SIZE(x) / PAGE_SIZE)
 
-#define KVM_PERMILLE_MMU_PAGES 20
+#define KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO 50
 #define KVM_MIN_ALLOC_MMU_PAGES 64UL
 #define KVM_MMU_HASH_SHIFT 12
 #define KVM_NUM_MMU_PAGES (1 << KVM_MMU_HASH_SHIFT)
@@ -1590,7 +1590,6 @@ void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
 				   const struct kvm_memory_slot *memslot);
 void kvm_mmu_zap_all(struct kvm *kvm);
 void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, u64 gen);
-unsigned long kvm_mmu_calculate_default_mmu_pages(struct kvm *kvm);
 void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned long kvm_nr_mmu_pages);
 
 int load_pdptrs(struct kvm_vcpu *vcpu, unsigned long cr3);
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 63c04c25fd66..64b6181f7757 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6131,30 +6131,6 @@ int kvm_mmu_module_init(void)
 	return ret;
 }
 
-/*
- * Calculate mmu pages needed for kvm.
- */
-unsigned long kvm_mmu_calculate_default_mmu_pages(struct kvm *kvm)
-{
-	unsigned long nr_mmu_pages;
-	unsigned long nr_pages = 0;
-	struct kvm_memslots *slots;
-	struct kvm_memory_slot *memslot;
-	int i;
-
-	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
-		slots = __kvm_memslots(kvm, i);
-
-		kvm_for_each_memslot(memslot, slots)
-			nr_pages += memslot->npages;
-	}
-
-	nr_mmu_pages = nr_pages * KVM_PERMILLE_MMU_PAGES / 1000;
-	nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
-
-	return nr_mmu_pages;
-}
-
 void kvm_mmu_destroy(struct kvm_vcpu *vcpu)
 {
 	kvm_mmu_unload(vcpu);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 13048683db60..5c770447f8c5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11833,9 +11833,13 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 				enum kvm_mr_change change)
 {
 	if (!kvm->arch.n_requested_mmu_pages &&
-	    (change == KVM_MR_CREATE || change == KVM_MR_DELETE))
-		kvm_mmu_change_mmu_pages(kvm,
-				kvm_mmu_calculate_default_mmu_pages(kvm));
+	    (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
+		unsigned long nr_mmu_pages;
+
+		nr_mmu_pages = kvm->nr_memslot_pages / KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO;
+		nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
+		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
+	}
 
 	kvm_mmu_slot_apply_flags(kvm, old, new, change);