linux-kernel - Re: [PATCH 2/9] KVM: x86: simplify read

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Date:	Fri, 20 Jul 2012 21:15:44 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/9] KVM: x86: simplify read_emulated

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[].

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