From mboxrd@z Thu Jan  1 00:00:00 1970
From: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Subject: Re: [PATCH 2/7] KVM: MMU: document clear_spte_count
Date: Wed, 19 Jun 2013 19:53:10 +0800
Message-ID: <51C19BA6.2060501@linux.vnet.ibm.com>
References: <1371632965-20077-1-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <1371632965-20077-3-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <51C196E9.2080508@redhat.com>
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Cc: gleb@redhat.com, avi.kivity@gmail.com, mtosatti@redhat.com,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
To: Paolo Bonzini <pbonzini@redhat.com>
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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.