From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753669Ab2GTNQI (ORCPT <rfc822;w@1wt.eu>);
	Fri, 20 Jul 2012 09:16:08 -0400
Received: from e23smtp03.au.ibm.com ([202.81.31.145]:43883 "EHLO
	e23smtp03.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753511Ab2GTNQG (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 20 Jul 2012 09:16:06 -0400
Message-ID: <50095A00.7010606@linux.vnet.ibm.com>
Date: Fri, 20 Jul 2012 21:15:44 +0800
From: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120615 Thunderbird/13.0.1
MIME-Version: 1.0
To: Marcelo Tosatti <mtosatti@redhat.com>
CC: Avi Kivity <avi@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        KVM <kvm@vger.kernel.org>
Subject: Re: [PATCH 2/9] KVM: x86: simplify read_emulated
References: <50056DB8.7080702@linux.vnet.ibm.com> <50056DE6.6020801@linux.vnet.ibm.com> <20120719235828.GA7121@amt.cnet> <5008BFC0.4070703@linux.vnet.ibm.com> <20120720105840.GA16859@amt.cnet>
In-Reply-To: <20120720105840.GA16859@amt.cnet>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
x-cbid: 12072013-6102-0000-0000-000001E8644C
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 07/20/2012 06:58 PM, Marcelo Tosatti wrote:
> On Fri, Jul 20, 2012 at 10:17:36AM +0800, Xiao Guangrong wrote:
>> On 07/20/2012 07:58 AM, Marcelo Tosatti wrote:
>>
>>>> -	}
>>>> +	rc = ctxt->ops->read_emulated(ctxt, addr, mc->data + mc->end, size,
>>>> +				      &ctxt->exception);
>>>> +	if (rc != X86EMUL_CONTINUE)
>>>> +		return rc;
>>>> +
>>>> +	mc->end += size;
>>>> +
>>>> +read_cached:
>>>> +	memcpy(dest, mc->data + mc->pos, size);
>>>
>>> What prevents read_emulated(size > 8) call, with
>>> mc->pos == (mc->end - 8) now?
>>
>> Marcelo,
>>
>> The splitting has been done in emulator_read_write_onepage:
>>
>> 	while (bytes) {
>> 		unsigned now = min(bytes, 8U);
>>
>> 		frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
>> 		frag->gpa = gpa;
>> 		frag->data = val;
>> 		frag->len = now;
>> 		frag->write_readonly_mem = (ret == -EPERM);
>>
>> 		gpa += now;
>> 		val += now;
>> 		bytes -= now;
>> 	}
>>
>> So i think it is safe to remove the splitting in read_emulated.
> 
> Yes, it is fine to remove it.
> 
> But splitting in emulate.c prevented the case of _cache read_ with size
>> 8 beyond end of mc->data. Must handle that case in read_emulated.
> 
> "What prevents read_emulated(size > 8) call, with mc->pos == (mc->end - 8) now?"

You mean the mmio region is partly cached?

I think it can not happen. Now, we pass the whole size to emulator_read_write_onepage(),
after it is finished, it saves the whole data into mc->data[], so, the cache-read
can always get the whole data from mc->data[].