KVM: VMX: Clean up DR6 emulation

As we trap all debug register accesses, we do not need to switch real
DR6 at all. Clean up update_exception_bitmap at this chance, too.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/vmx.c

@@ -579,17 +579,12 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
 {
 	u32 eb;
 
-	eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR)
-		| (1u << NM_VECTOR);
-	/*
-	 * Unconditionally intercept #DB so we can maintain dr6 without
-	 * reading it every exit.
-	 */
-	eb |= 1u << DB_VECTOR;
-	if (vcpu->guest_debug & KVM_GUESTDBG_ENABLE) {
-		if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
+	eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR) |
+	     (1u << NM_VECTOR) | (1u << DB_VECTOR);
+	if ((vcpu->guest_debug &
+	     (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP)) ==
+	    (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP))
 		eb |= 1u << BP_VECTOR;
-	}
 	if (to_vmx(vcpu)->rmode.vm86_active)
 		eb = ~0;
 	if (enable_ept)
@@ -3777,9 +3772,6 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
 	 */
 	vmcs_writel(HOST_CR0, read_cr0());
 
-	if (vcpu->arch.switch_db_regs)
-		set_debugreg(vcpu->arch.dr6, 6);
-
 	asm(
 		/* Store host registers */
 		"push %%"R"dx; push %%"R"bp;"
@@ -3880,9 +3872,6 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
 				  | (1 << VCPU_EXREG_PDPTR));
 	vcpu->arch.regs_dirty = 0;
 
-	if (vcpu->arch.switch_db_regs)
-		get_debugreg(vcpu->arch.dr6, 6);
-
 	vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
 	if (vmx->rmode.irq.pending)
 		fixup_rmode_irq(vmx);