Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

Xiao Guangrong
2013-12-03T07:11:07Z

On 11/28/2013 04:53 PM, Xiao Guangrong wrote:
> On 11/27/2013 03:31 AM, Marcelo Tosatti wrote:
>> On Tue, Nov 26, 2013 at 11:21:37AM +0800, Xiao Guangrong wrote:
>>> On 11/26/2013 02:12 AM, Marcelo Tosatti wrote:
>>>> On Mon, Nov 25, 2013 at 02:29:03PM +0800, Xiao Guangrong wrote:
>>>>>>> Also, there is no guarantee of termination (as long as sptes are
>>>>>>> deleted with the correct timing). BTW, can't see any guarantee of
>>>>>>> termination for rculist nulls either (a writer can race with a lockless
>>>>>>> reader indefinately, restarting the lockless walk every time).
>>>>>>
>>>>>> Hmm, that can be avoided by checking dirty-bitmap before rewalk,
>>>>>> that means, if the dirty-bitmap has been set during lockless write-protection,
>>>>>> it�s unnecessary to write-protect its sptes. Your idea?
>>>>> This idea is based on the fact that the number of rmap is limited by
>>>>> RMAP_RECYCLE_THRESHOLD. So, in the case of adding new spte into rmap,
>>>>> we can break the rewalk at once, in the case of deleting, we can only
>>>>> rewalk RMAP_RECYCLE_THRESHOLD times.
>>>>
>>>> Please explain in more detail.
>>>
>>> Okay.
>>>
>>> My proposal is like this:
>>>
>>> pte_list_walk_lockless()
>>> {
>>> restart:
>>>
>>> + if (__test_bit(slot->arch.dirty_bitmap, gfn-index))
>>> + return;
>>>
>>> code-doing-lockless-walking;
>>> ......
>>> }
>>>
>>> Before do lockless-walking, we check the dirty-bitmap first, if
>>> it is set we can simply skip write-protection for the gfn, that
>>> is the case that new spte is being added into rmap when we lockless
>>> access the rmap.
>>
>> The dirty bit could be set after the check.
>>
>>> For the case of deleting spte from rmap, the number of entry is limited
>>> by RMAP_RECYCLE_THRESHOLD, that is not endlessly.
>>
>> It can shrink and grow while lockless walk is performed.
>
> Yes, indeed.
>
> Hmmm, another idea in my mind to fix this is encoding the position into
> the reserved bits of desc->more pointer, for example:
>
> +------+ +------+ +------+
> rmapp -> |Desc 0| -> |Desc 1| -> |Desc 2|
> +------+ +------+ +------+
>
> There are 3 descs on the rmap, and:
> rmapp = &desc0 | 1UL | 3UL << 50;
> desc0->more = desc1 | 2UL << 50;
> desc1->more = desc0 | 1UL << 50
> desc2->more = &rmapp | 1UL; (The nulls pointer)
>
> We will walk to the next desc only if the "position" of current desc
> is >= the position of next desc. That can make sure we can reach the
> last desc anyway.
>
> And in order to avoiding doing too many "rewalk", we will goto the
> slow path (do walk with holding the lock) instead when retry the walk
> more that N times.

How about this idea? Or you guys still prefer to the idea of lockless on
first-level?