On 06/19/2013 07:55 PM, Paolo Bonzini wrote:
> 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.
In order to detect repeatedly marking spte present to stop the lockless side
to see present to present change, otherwise, we can get this:
Say spte = 0xa11110001 (high 32bits = 0xa, low 32bit = 0x11110001)
CPU 1: __get_spte_lockless CPU 2: __update_clear_spte_slow
----------------------------------------------------------------------
read low: low= 0x11110001
clear the spte, then spte = 0x0ull
read high: high = 0x0
set spte to 0xb11110001 (high 32bits = 0xb,
low 32bit = 0x11110001)
read low: 0x11110001 and see
it is not changed.
In this case, CPU 1 see the low bits are not changed, then it tries to access the memory at:
0x11110000.
BTW, we are using tlb to protect lockless walking, the count can be drop after
improving kvm_set_pte_rmapp where is the only place change spte from present to present
without TLB flush.
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