Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753489Ab2JVLxX (ORCPT <rfc822;w@1wt.eu>);
	Mon, 22 Oct 2012 07:53:23 -0400
Received: from goliath.siemens.de ([192.35.17.28]:17486 "EHLO
	goliath.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753412Ab2JVLxW (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 22 Oct 2012 07:53:22 -0400
Message-ID: <50852F9C.9020808@siemens.com>
Date: Mon, 22 Oct 2012 13:35:56 +0200
From: Jan Kiszka <jan.kiszka@siemens.com>
User-Agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666
MIME-Version: 1.0
To: Gleb Natapov <gleb@redhat.com>
CC: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>,
        Avi Kivity <avi@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, KVM <kvm@vger.kernel.org>
Subject: Re: [PATCH] KVM: x86: fix vcpu->mmio_fragments overflow
References: <5081033C.4060503@linux.vnet.ibm.com> <20121022091615.GG29310@redhat.com> <50852972.305@linux.vnet.ibm.com> <20121022112314.GO29310@redhat.com>
In-Reply-To: <20121022112314.GO29310@redhat.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2226
Lines: 51

On 2012-10-22 13:23, Gleb Natapov wrote:
> On Mon, Oct 22, 2012 at 07:09:38PM +0800, Xiao Guangrong wrote:
>> On 10/22/2012 05:16 PM, Gleb Natapov wrote:
>>> On Fri, Oct 19, 2012 at 03:37:32PM +0800, Xiao Guangrong wrote:
>>>> After commit b3356bf0dbb349 (KVM: emulator: optimize "rep ins" handling),
>>>> the pieces of io data can be collected and write them to the guest memory
>>>> or MMIO together.
>>>>
>>>> Unfortunately, kvm splits the mmio access into 8 bytes and store them to
>>>> vcpu->mmio_fragments. If the guest uses "rep ins" to move large data, it
>>>> will cause vcpu->mmio_fragments overflow
>>>>
>>>> The bug can be exposed by isapc (-M isapc):
>>>>
>>>> [23154.818733] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
>>>> [ ......]
>>>> [23154.858083] Call Trace:
>>>> [23154.859874]  [<ffffffffa04f0e17>] kvm_get_cr8+0x1d/0x28 [kvm]
>>>> [23154.861677]  [<ffffffffa04fa6d4>] kvm_arch_vcpu_ioctl_run+0xcda/0xe45 [kvm]
>>>> [23154.863604]  [<ffffffffa04f5a1a>] ? kvm_arch_vcpu_load+0x17b/0x180 [kvm]
>>>>
>>>>
>>>> Actually, we can use one mmio_fragment to store a large mmio access for the
>>>> mmio access is always continuous then split it when we pass the mmio-exit-info
>>>> to userspace. After that, we only need two entries to store mmio info for
>>>> the cross-mmio pages access
>>>>
>>> I wonder can we put the data into coalesced mmio buffer instead of
>>
>> If we put all mmio data into coalesced buffer, we should:
>> - ensure the userspace program uses KVM_REGISTER_COALESCED_MMIO to register
>>   all mmio regions.
>>
> It appears to not be so.
> Userspace calls kvm_flush_coalesced_mmio_buffer() after returning from
> KVM_RUN which looks like this:

Nope, no longer, only on accesses to devices that actually use such
regions (and there are only two ATM). The current design of a global
coalesced mmio ring is horrible /wrt latency.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/