linux-kernel - Re: [PATCH 2/7] KVM: MMU: document clear_spte

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Date:	Wed, 19 Jun 2013 19:53:10 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	gleb@...hat.com, avi.kivity@...il.com, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 2/7] KVM: MMU: document clear_spte_count

On 06/19/2013 07:32 PM, Paolo Bonzini wrote:
> Il 19/06/2013 11:09, Xiao Guangrong ha scritto:
>> Document it to Documentation/virtual/kvm/mmu.txt
> 
> While reviewing the docs, I looked at the code.
> 
> Why can't this happen?
> 
>     CPU 1: __get_spte_lockless          CPU 2: __update_clear_spte_slow
> ------------------------------------------------------------------------------
>                                         write low
>     read count
>     read low
>     read high
>                                         write high
>     check low and count
>                                         update count
> 
> The check passes, but CPU 1 read a "torn" SPTE.

In this case, CPU 1 will read the "new low bits" and the "old high bits", right?
the P bit in the low bits is cleared when do __update_clear_spte_slow, i.e, it is
not present, so the whole value is ignored.

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