KVM: x86/mmu: Buffer nested MMU split_desc_cache only by default capacity

Buffer split_desc_cache, the cache used to allcoate rmap list entries,
only by the default cache capacity (currently 40), not by doubling the
minimum (513).  Aliasing L2 GPAs to L1 GPAs is uncommon, thus eager page
splitting is unlikely to need 500+ entries.  And because each object is a
non-trivial 128 bytes (see struct pte_list_desc), those extra ~500
entries means KVM is in all likelihood wasting ~64kb of memory per VM.

Link: https://lore.kernel.org/all/YrTDcrsn0%2F+alpzf@google.com
Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220624171808.2845941-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -6118,17 +6118,25 @@ static bool need_topup_split_caches_or_resched(struct kvm *kvm)
 
 static int topup_split_caches(struct kvm *kvm)
 {
+	/*
+	 * Allocating rmap list entries when splitting huge pages for nested
+	 * MMUs is uncommon as KVM needs to allocate if and only if there is
+	 * more than one rmap entry for a gfn, i.e. requires an L1 gfn to be
+	 * aliased by multiple L2 gfns.  Aliasing gfns when using TDP is very
+	 * atypical for VMMs; a few gfns are often aliased during boot, e.g.
+	 * when remapping firmware, but aliasing rarely occurs post-boot).  If
+	 * there is only one rmap entry, rmap->val points directly at that one
+	 * entry and doesn't need to allocate a list.  Buffer the cache by the
+	 * default capacity so that KVM doesn't have to topup the cache if it
+	 * encounters an aliased gfn or two.
+	 */
+	const int capacity = SPLIT_DESC_CACHE_MIN_NR_OBJECTS +
+			     KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE;
 	int r;
 
 	lockdep_assert_held(&kvm->slots_lock);
 
-	/*
-	 * Setting capacity == min would cause KVM to drop mmu_lock even if
-	 * just one object was consumed from the cache, so make capacity
-	 * larger than min.
-	 */
-	r = __kvm_mmu_topup_memory_cache(&kvm->arch.split_desc_cache,
-					 2 * SPLIT_DESC_CACHE_MIN_NR_OBJECTS,
+	r = __kvm_mmu_topup_memory_cache(&kvm->arch.split_desc_cache, capacity,
 					 SPLIT_DESC_CACHE_MIN_NR_OBJECTS);
 	if (r)
 		return r;