KVM: x86: Don't attempt to load PDPTRs when 64-bit mode is enabled

Don't attempt to load PDPTRs if EFER.LME=1, i.e. if 64-bit mode is
enabled.  A recent change to reload the PDTPRs when CR0.CD or CR0.NW is
toggled botched the EFER.LME handling and sends KVM down the PDTPR path
when is_paging() is true, i.e. when the guest toggles CD/NW in 64-bit
mode.

Split the CR0 checks for 64-bit vs. 32-bit PAE into separate paths.  The
64-bit path is specifically checking state when paging is toggled on,
i.e. CR0.PG transititions from 0->1.  The PDPTR path now needs to run if
the new CR0 state has paging enabled, irrespective of whether paging was
already enabled.  Trying to shave a few cycles to make the PDPTR path an
"else if" case is a mess.

Fixes: d42e3fae ("kvm: x86: Read PDPTEs on CR0.CD and CR0.NW changes")
Cc: Jim Mattson <jmattson@google.com>
Cc: Oliver Upton <oupton@google.com>
Cc: Peter Shier <pshier@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20200714015732.32426-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -820,9 +820,9 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
 	if ((cr0 & X86_CR0_PG) && !(cr0 & X86_CR0_PE))
 		return 1;
 
-	if (cr0 & X86_CR0_PG) {
 #ifdef CONFIG_X86_64
-		if (!is_paging(vcpu) && (vcpu->arch.efer & EFER_LME)) {
+	if ((vcpu->arch.efer & EFER_LME) && !is_paging(vcpu) &&
+	    (cr0 & X86_CR0_PG)) {
 		int cs_db, cs_l;
 
 		if (!is_pae(vcpu))
@@ -830,12 +830,12 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
 		kvm_x86_ops.get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
 		if (cs_l)
 			return 1;
-		} else
+	}
 #endif
-		if (is_pae(vcpu) && ((cr0 ^ old_cr0) & pdptr_bits) &&
+	if (!(vcpu->arch.efer & EFER_LME) && (cr0 & X86_CR0_PG) &&
+	    is_pae(vcpu) && ((cr0 ^ old_cr0) & pdptr_bits) &&
 	    !load_pdptrs(vcpu, vcpu->arch.walk_mmu, kvm_read_cr3(vcpu)))
 		return 1;
-	}
 
 	if (!(cr0 & X86_CR0_PG) && kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE))
 		return 1;