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

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Date:	Tue, 10 Jul 2012 13:45:15 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.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 07/10/2012 01:36 PM, Gleb Natapov wrote:
> On Mon, Jul 09, 2012 at 04:34:50PM +0300, Avi Kivity wrote:
> > 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.
> > 
> But the code is already here, why drop it?

The read cache is not effective for multiple disjunct reads.  The
splitting into 8-byte groups is unneeded.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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