Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault

Xiao Guangrong
2012-05-07T06:52:40Z

On 05/05/2012 10:08 PM, Marcelo Tosatti wrote:

>>
>> I am confused with ' _everywhere_ ', it means all of the path read/update
>> spte? why not only verify the path which depends on is_writable_pte()?
>
> I meant any path that updates from present->present.
>

OK, got it. So let us focus on mmu_spte_update() only. :)

>> For the reason of "its easy to verify that it is correct"? But these
>> paths are safe since it is not care PT_WRITABLE_MASK at all. What these
>> paths care is the Dirty-bit and Accessed-bit are not lost, that is why
>> we always treat the spte as "volatile" if it is can be updated out of
>> mmu-lock.
>>
>> For the further development? We can add the delta comment for
>> is_writable_pte() to warn the developer use it more carefully.
>>
>> It is also very hard to verify spte everywhere. :(
>>
>> Actually, the current code to care PT_WRITABLE_MASK is just for
>> tlb flush, may be we can fold it into mmu_spte_update.
>> [
>> There are tree ways to modify spte, present -> nonpresent, nonpresent -> present,
>> present -> present.
>>
>> But we only need care present -> present for lockless.
>> ]
>
> Also need to take memory ordering into account, which was not an issue
> before. So it is not only TLB flush.

It seems do not need explicit barrier, we always use atomic-xchg to update
spte, it has already guaranteed the memory ordering.

In mmu_spte_update():

/* the return value indicates wheater tlb need be flushed.*/
static bool mmu_spte_update(u64 *sptep, u64 new_spte)
{
u64 old_spte = *sptep;
bool flush = false;

old_spte = xchg(sptep, new_spte);

if (is_writable_pte(old_spte) && !is_writable_pte(spte) )
flush = true;

.....
}

>
>> /*
>> * return true means we need flush tlbs caused by changing spte from writeable
>> * to read-only.
>> */
>> bool mmu_update_spte(u64 *sptep, u64 spte)
>> {
>> u64 last_spte, old_spte = *sptep;
>> bool flush = false;
>>
>> last_spte = xchg(sptep, spte);
>>
>> if ((is_writable_pte(last_spte) ||
>> spte_has_updated_lockless(old_spte, last_spte)) &&
>> !is_writable_pte(spte) )
>> flush = true;
>>
>> .... track Drity/Accessed bit ...
>>
>>
>> return flush
>> }
>>
>> Furthermore, the style of "if (spte-has-changed) goto beginning" is feasible
>> in set_spte since this path is a fast path. (i can speed up mmu_need_write_protect)
>
> What you mean exactly?
>
> It would be better if all these complications introduced by lockless
> updates can be avoided, say using A/D bits as Avi suggested.

Anyway, i do not object it if we have a better way to do these, but ......