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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 19/06/2013 13:53, Xiao Guangrong ha scritto:
> 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.

Indeed that's what the comment says, too.  But then why do we need the
count at all?  The spte that is read is exactly the same before and
after the count is updated.

Paolo