Re: [PATCH 00/13] KVM: MMU: fast page fault

[Xiao Guangrong]

On 04/10/2012 03:46 AM, Marcelo Tosatti wrote:

> On Tue, Apr 10, 2012 at 02:26:27AM +0800, Xiao Guangrong wrote:
>> On 04/10/2012 01:58 AM, Marcelo Tosatti wrote:
>>
>>> On Mon, Apr 09, 2012 at 04:12:46PM +0300, Avi Kivity wrote:
>>>> On 03/29/2012 11:20 AM, Xiao Guangrong wrote:
>>>>> * Idea
>>>>> The present bit of page fault error code (EFEC.P) indicates whether the
>>>>> page table is populated on all levels, if this bit is set, we can know
>>>>> the page fault is caused by the page-protection bits (e.g. W/R bit) or
>>>>> the reserved bits.
>>>>>
>>>>> In KVM, in most cases, all this kind of page fault (EFEC.P = 1) can be
>>>>> simply fixed: the page fault caused by reserved bit
>>>>> (EFFC.P = 1 && EFEC.RSV = 1) has already been filtered out in fast mmio
>>>>> path. What we need do to fix the rest page fault (EFEC.P = 1 && RSV != 1)
>>>>> is just increasing the corresponding access on the spte.
>>>>>
>>>>> This pachset introduces a fast path to fix this kind of page fault: it
>>>>> is out of mmu-lock and need not walk host page table to get the mapping
>>>>> from gfn to pfn.
>>>>>
>>>>>
>>>>
>>>> This patchset is really worrying to me.
>>>>
>>>> It introduces a lot of concurrency into data structures that were not
>>>> designed for it.  Even if it is correct, it will be very hard to
>>>> convince ourselves that it is correct, and if it isn't, to debug those
>>>> subtle bugs.  It will also be much harder to maintain the mmu code than
>>>> it is now.
>>>>
>>>> There are a lot of things to check.  Just as an example, we need to be
>>>> sure that if we use rcu_dereference() twice in the same code path, that
>>>> any inconsistencies due to a write in between are benign.  Doing that is
>>>> a huge task.
>>>>
>>>> But I appreciate the performance improvement and would like to see a
>>>> simpler version make it in.  This needs to reduce the amount of data
>>>> touched in the fast path so it is easier to validate, and perhaps reduce
>>>> the number of cases that the fast path works on.
>>>>
>>>> I would like to see the fast path as simple as
>>>>
>>>>   rcu_read_lock();
>>>>
>>>>   (lockless shadow walk)
>>>>   spte = ACCESS_ONCE(*sptep);
>>>>
>>>>   if (!(spte & PT_MAY_ALLOW_WRITES))
>>>>         goto slow;
>>>>
>>>>   gfn = kvm_mmu_page_get_gfn(sp, sptep - sp->sptes)
>>>>   mark_page_dirty(kvm, gfn);
>>>>
>>>>   new_spte = spte & ~(PT64_MAY_ALLOW_WRITES | PT_WRITABLE_MASK);
>>>>   if (cmpxchg(sptep, spte, new_spte) != spte)
>>>>        goto slow;
>>>>
>>>>   rcu_read_unlock();
>>>>   return;
>>>>
>>>> slow:
>>>>   rcu_read_unlock();
>>>>   slow_path();
>>>>
>>>> It now becomes the responsibility of the slow path to maintain *sptep &
>>>> PT_MAY_ALLOW_WRITES, but that path has a simpler concurrency model.  It
>>>> can be as simple as a clear_bit() before we update sp->gfns[] or if we
>>>> add host write protection.
>>>>
>>>> Sorry, it's too complicated for me.  Marcelo, what's your take?
>>>
>>> The improvement is small and limited to special cases (migration should
>>> be rare and framebuffer memory accounts for a small percentage of total
>>> memory).
>>
>>
>> Actually, although the framebuffer is small but it is modified really
>> frequently, and another unlucky things is that dirty-log is also
>> very frequently and need hold mmu-lock to do write-protect.
>>
>> Yes, if Xwindow is not enabled, the benefit is limited. :)
> 
> Ignoring that fact, the safety of lockless set_spte and friends is not
> clear.
> 


That is why AVI suggested me to simplify the whole things. :)


> Perhaps the mmu_lock hold times by get_dirty are a large component here?
> If that can be alleviated, not only RO->RW faults benefit.
> 

Yes.

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