linux-kernel - Re: [PATCH 1/2] KVM: X86: remove read buffer for mmio read

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Message-ID: <4FFADDFA.5000603@redhat.com>
Date:	Mon, 09 Jul 2012 16:34:50 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	Gleb Natapov <gleb@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: X86: remove read buffer for mmio read

On 07/09/2012 04:23 PM, Xiao Guangrong wrote:
> On 07/09/2012 08:49 PM, Avi Kivity wrote:
>> On 07/09/2012 02:23 PM, Gleb Natapov wrote:
>>>
>>>> kvm-unit-tests.git has a test for xchg to mmio.  Does it still work?
>>>>
>>>> I agree this code has to go, but it needs to be replaced by something.
>>>> Maybe a .valid flag in struct operand.
>>>>
>>> Valid will not enough for that.
>> 
>> If we make everything go through operands, any reason why not?
>> 
> 
> I noticed some instructions need to read ESP for many times (e.g, iret_real),
> maybe .valid flag is not enough for this case if the stack is in MMIO, yes?

Good catch.  We either have to fix it or to restrict stack operations to
regular memory (->read_std).

> IIUC, I also noticed ESP is not reset back if it is emulated fail (mmio is needed).
> If the stack located in mmio region, this kind of instruct will be broken, i know no
> guest will use mmio as stack but SDM does not limit it, is it valid?

Stack in mmio (or task switch in mmio) is architecturally valid.  We
don't have to support it if no guests do it.

-- 
error compiling committee.c: too many arguments to function


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