From mboxrd@z Thu Jan  1 00:00:00 1970
From: Nadav Amit <nadav.amit@gmail.com>
Subject: x86: Question regarding the reset value of LINT0
Date: Wed, 8 Apr 2015 19:40:32 +0300
Message-ID: <2B474EEE-85C9-47C3-89FF-C56754CFEC0D@gmail.com>
Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2070.6\))
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: QUOTED-PRINTABLE
To: kvm list <kvm@vger.kernel.org>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mail-wg0-f45.google.com ([74.125.82.45]:33046 "EHLO
	mail-wg0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753364AbbDHQkg convert rfc822-to-8bit (ORCPT
	<rfc822;kvm@vger.kernel.org>); Wed, 8 Apr 2015 12:40:36 -0400
Received: by wgin8 with SMTP id n8so94265873wgi.0
        for <kvm@vger.kernel.org>; Wed, 08 Apr 2015 09:40:35 -0700 (PDT)
Received: from nadav-mbp.cs.technion.ac.il ([132.68.46.112])
        by mx.google.com with ESMTPSA id hj10sm16331858wjc.48.2015.04.08.09.40.33
        for <kvm@vger.kernel.org>
        (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128);
        Wed, 08 Apr 2015 09:40:33 -0700 (PDT)
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

Hi,

I would appreciate if someone explains the reason for enabling LINT0 du=
ring
APIC reset. This does not correspond with Intel SDM Figure 10-8: =E2=80=
=9CLocal
Vector Table=E2=80=9D that says all LVT registers are reset to 0x10000.

In kvm_lapic_reset, I see:

	apic_set_reg(apic, APIC_LVT0,
		SET_APIC_DELIVERY_MODE(0, APIC_MODE_EXTINT));

Which is actually pretty similar to QEMU=E2=80=99s apic_reset_common:

    if (bsp) {
        /*
         * LINT0 delivery mode on CPU #0 is set to ExtInt at initializa=
tion
         * time typically by BIOS, so PIC interrupt can be delivered to=
 the
         * processor when local APIC is enabled.
         */
        s->lvt[APIC_LVT_LINT0] =3D 0x700;
    }

Yet, in both cases, I miss the point - if it is typically done by the B=
IOS,
why does QEMU or KVM enable it?

BTW: KVM seems to run fine without it, and I think setting it causes me
problems in certain cases.

Thanks,
Nadav