Re: [PATCH 2/7] KVM: MMU: document clear_spte_count

From: Xiao Guangrong
Date: Wed Jun 19 2013 - 07:53:29 EST

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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