KVM: MMU: improve n_max_mmu_pages calculation with TDP

[Marcelo Tosatti <mtosatti@redhat.com>]

kvm_mmu_calculate_mmu_pages numbers, 

maximum number of shadow pages = 2% of mapped guest pages

Does not make sense for TDP guests where mapping all of guest
memory with 4k pages cannot exceed "mapped guest pages / 512"
(not counting root pages).

Allow that maximum for TDP, forcing the guest to recycle otherwise.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 956ca35..a9694a8d7 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4293,7 +4293,7 @@ nomem:
 unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm)
 {
 	unsigned int nr_mmu_pages;
-	unsigned int  nr_pages = 0;
+	unsigned int i, nr_pages = 0;
 	struct kvm_memslots *slots;
 	struct kvm_memory_slot *memslot;
 
@@ -4302,7 +4302,19 @@ unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm)
 	kvm_for_each_memslot(memslot, slots)
 		nr_pages += memslot->npages;
 
-	nr_mmu_pages = nr_pages * KVM_PERMILLE_MMU_PAGES / 1000;
+	if (tdp_enabled) {
+		/* one root page */
+		nr_mmu_pages = 1;
+		/* nr_pages / (512^i) per level, due to
+		 * guest RAM map being linear */
+		for (i = 1; i < 4; i++) {
+			int nr_pages_round = nr_pages + (1 << (9*i));
+			nr_mmu_pages += nr_pages_round >> (9*i);
+		}
+	} else {
+		nr_mmu_pages = nr_pages * KVM_PERMILLE_MMU_PAGES / 1000;
+	}
+
 	nr_mmu_pages = max(nr_mmu_pages,
 			(unsigned int) KVM_MIN_ALLOC_MMU_PAGES);