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

[Xiao Guangrong]

On 06/19/2013 08:41 PM, Paolo Bonzini wrote:
> Il 19/06/2013 14:25, Xiao Guangrong ha scritto:
>> 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.
> 
> Got it.  What about this in the comment to __get_spte_lockless:
> 
>  * The idea using the light way get the spte on x86_32 guest is from
>  * gup_get_pte(arch/x86/mm/gup.c).
>  *
>  * An spte tlb flush may be pending, because kvm_set_pte_rmapp
>  * coalesces them and we are running out of the MMU lock.  Therefore
>  * we need to protect against in-progress updates of the spte.
>  *
>  * A race on changing present->non-present may get the old value for
>  * the high part of the spte.  This is okay because the high part of
>  * the spte is ignored for non-present spte.
>  *
>  * However, we must detect a present->present change and reread the
>  * spte in case the change is in progress.  Because all such changes
>  * are done in two steps (present->non-present and non-present->present),
>  * it is enough to count the number of present->non-present updates,
>  * which is done using clear_spte_count.

It is fantastic :)

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