Revert "kvm: x86: optimize dr6 restore"

This reverts commit 0e0a53c5.

As Christian Ehrhardt noted:

  The most common case is that vcpu->arch.dr6 and the host's %dr6 value
  are not related at all because ->switch_db_regs is zero. To do this
  all correctly, we must handle the case where the guest leaves an arbitrary
  unused value in vcpu->arch.dr6 before disabling breakpoints again.

  However, this means that vcpu->arch.dr6 is not suitable to detect the
  need for a %dr6 clear.
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/x86.c

@@ -3271,15 +3271,10 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 	kvm_x86_ops->vcpu_put(vcpu);
 	vcpu->arch.last_host_tsc = rdtsc();
 	/*
-	 * Here dr6 is either zero or, if the guest has run and userspace
-	 * has not set any breakpoints or watchpoints, it can be set to
-	 * the guest dr6 (stored in vcpu->arch.dr6). do_debug expects dr6
-	 * to be cleared after it runs, so clear the host register.  However,
-	 * MOV to DR can be expensive when running nested, omit it if
-	 * vcpu->arch.dr6 is already zero: in that case, the host dr6 cannot
-	 * currently be nonzero.
-	 */
-	if (vcpu->arch.dr6)
+	 * If userspace has set any breakpoints or watchpoints, dr6 is restored
+	 * on every vmexit, but if not, we might have a stale dr6 from the
+	 * guest. do_debug expects dr6 to be cleared after it runs, do the same.
+	 */
 	set_debugreg(0, 6);
 }