KVM: x86/mmu: Account NX huge page disallowed iff huge page was requested

Condition the accounting of a disallowed huge NX page on the original
requested level of the page being greater than the current iterator
level.  This does two things: accounts the page if and only if a huge
page was actually disallowed, and accounts the shadow page if and only
if it was the level at which the huge page was disallowed.  For the
latter case, the previous logic would account all shadow pages used to
create the translation for the forced small page, e.g. even PML4, which
can't be a huge page on current hardware, would be accounted as having
been a disallowed huge page when using 5-level EPT.

The overzealous accounting is purely a performance issue, i.e. the
recovery thread will spuriously zap shadow pages, but otherwise the bad
behavior is harmless.

Cc: Junaid Shahid <junaids@google.com>
Fixes: b8e8c830 ("kvm: mmu: ITLB_MULTIHIT mitigation")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923183735.584-6-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -3383,7 +3383,8 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
 					      it.level - 1, true, ACC_ALL);
 
 			link_shadow_page(vcpu, it.sptep, sp);
-			if (is_tdp && huge_page_disallowed)
+			if (is_tdp && huge_page_disallowed &&
+			    req_level >= it.level)
 				account_huge_nx_page(vcpu->kvm, sp);
 		}
 	}

arch/x86/kvm/mmu/paging_tmpl.h

@@ -708,7 +708,7 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, gpa_t addr,
 			sp = kvm_mmu_get_page(vcpu, base_gfn, addr,
 					      it.level - 1, true, direct_access);
 			link_shadow_page(vcpu, it.sptep, sp);
-			if (huge_page_disallowed)
+			if (huge_page_disallowed && req_level >= it.level)
 				account_huge_nx_page(vcpu->kvm, sp);
 		}
 	}