KVM: x86: MMU: Initialize force_pt_level before calling mapping_level()

Commit fd136902 ("KVM: x86: MMU: Move mapping_level_dirty_bitmap()
call in mapping_level()") forgot to initialize force_pt_level to false
in FNAME(page_fault)() before calling mapping_level() like
nonpaging_map() does.  This can sometimes result in forcing page table
level mapping unnecessarily.

Fix this and move the first *force_pt_level check in mapping_level()
before kvm_vcpu_gfn_to_memslot() call to make it a bit clearer that
the variable must be initialized before mapping_level() gets called.

This change can also avoid calling kvm_vcpu_gfn_to_memslot() when
!check_hugepage_cache_consistency() check in tdp_page_fault() forces
page table level mapping.
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.c

@@ -886,9 +886,10 @@ static int mapping_level(struct kvm_vcpu *vcpu, gfn_t large_gfn,
 	int host_level, level, max_level;
 	struct kvm_memory_slot *slot;
 
-	slot = kvm_vcpu_gfn_to_memslot(vcpu, large_gfn);
+	if (unlikely(*force_pt_level))
+		return PT_PAGE_TABLE_LEVEL;
 
-	if (likely(!*force_pt_level))
+	slot = kvm_vcpu_gfn_to_memslot(vcpu, large_gfn);
 	*force_pt_level = !memslot_valid_for_gpte(slot, true);
 	if (unlikely(*force_pt_level))
 		return PT_PAGE_TABLE_LEVEL;

arch/x86/kvm/paging_tmpl.h

@@ -698,7 +698,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
 	int r;
 	pfn_t pfn;
 	int level = PT_PAGE_TABLE_LEVEL;
-	bool force_pt_level;
+	bool force_pt_level = false;
 	unsigned long mmu_seq;
 	bool map_writable, is_self_change_mapping;