Il 29/07/2013 15:27, Gleb Natapov ha scritto:
>>>> doesn't look bad at all. With the old check on EPT it looked ugly, but
>>>> with the new check on PT_GUEST_DIRTY_MASK it is quite natural. Also
>>>> because you have anyway a reference to PT_GUEST_DIRTY_MASK in the "if".
>>>> If I see
>>>>
>>>> if (!write_fault)
>>>> protect_clean_gpte(&pte_access, pte);
>>>> else
>>>> /*
>>>> * On a write fault, fold the dirty bit into
>>>> * accessed_dirty by
>>>> * shifting it one place right.
>>>> */
>>>> accessed_dirty &=
>>>> pte >> (PT_DIRTY_SHIFT - PT_ACCESSED_SHIFT);
>>>>
>>>> if (PT_GUEST_DIRTY_MASK != 0 && unlikely(!accessed_dirty)) {
>>>>
>>>> the obvious reaction is "what, is there a case where I'm using
>>>> accessed_dirty if PT_GUEST_DIRTY_MASK == 0?" Of course it makes sense
>>> In this case accessed_dirty has correct value of 0 :) The if() bellow just
>>> tells you that since A/D is not supported there is nothing to be done
>>> about zero value of accessed_dirty, but the value itself is correct!
>>
>> It is correct because accessed_dirty is initialized to 0. But the "&"
>> with a bit taken out of thin air (bit 0 of the PTE)? That's just
>> disgusting. :)
>>
> Sorry to disgust you, but the code relies on this "&" trick with or
> without the patch. It clears all unrelated bits from pte this way. No
> new disgusting tricks are added by the patch.
Oh the code is not disgusting at all! It is very nice to follow.
The new disgusting ;) trick is that here in the EPT case you're
effectively doing
accessed_dirty &= pte;
where bit 0 is the "R" bit (iirc) and has absolutely nothing to do with
dirty or accessed.
Paolo
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