From mboxrd@z Thu Jan  1 00:00:00 1970
From: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Subject: Re: [PATCH RFC 5/5] KVM: This is the main part of the "moving dirty
 bitmaps to user space"
Date: Tue, 13 Apr 2010 05:55:10 +0900
Message-ID: <4BC388AE.9010903@oss.ntt.co.jp>
References: <20100409182732.857de4db.yoshikawa.takuya@oss.ntt.co.jp> <20100409183808.b72fc9a3.yoshikawa.takuya@oss.ntt.co.jp> <4BC2051B.9090101@redhat.com> <4BC28578.4050102@oss.ntt.co.jp> <4BC2E64E.1090202@redhat.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Cc: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>,
	mtosatti@redhat.com, kvm@vger.kernel.org
To: Avi Kivity <avi@redhat.com>
Return-path: <kvm-owner@vger.kernel.org>
Received: from serv2.oss.ntt.co.jp ([222.151.198.100]:58129 "EHLO
	serv2.oss.ntt.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753898Ab0DLVS3 (ORCPT <rfc822;kvm@vger.kernel.org>);
	Mon, 12 Apr 2010 17:18:29 -0400
In-Reply-To: <4BC2E64E.1090202@redhat.com>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

Avi Kivity wrote:
> On 04/12/2010 05:29 AM, Takuya Yoshikawa wrote:
>>>> TODO:
>>>> 1. We want to use copy_in_user() for 32bit case too.
>>>
>>> Definitely. Why doesn't it work now?
>>
>> Sadly we don't have that for 32bit. We have to implement by ourselves.
>>
>> I tested two temporary implementations for 32bit:
>>   1. This version using copy_from_user() and copy_to_user() with
>>      not nice vmalloc().
>>   2. Loop with __get_user() and __put_user().
>>
>> The result was 1 is much faster than 2.
>
> What about copy_from_user()/copy_to_user() through a 512 byte buffer 
> on the kernel stack?

Reserving 512 bytes on the stack looks like too much, I'd rather kmalloc 
a 512 byte buffer at VM
creation time and pass it down to the dirty page tracking code. Would 
you be OK with such an
approach?