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

[Xiao Guangrong]

On 11/25/2013 02:11 PM, Xiao Guangrong wrote:
> 
> On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> 
>> On Wed, Oct 23, 2013 at 09:29:25PM +0800, Xiao Guangrong wrote:
>>> It likes nulls list and we use the pte-list as the nulls which can help us to
>>> detect whether the "desc" is moved to anther rmap then we can re-walk the rmap
>>> if that happened
>>>
>>> kvm->slots_lock is held when we do lockless walking that prevents rmap
>>> is reused (free rmap need to hold that lock) so that we can not see the same
>>> nulls used on different rmaps
>>>
>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
>>
>> How about simplified lockless walk on the slot while rmapp entry
>> contains a single spte? (which should be the case with two-dimensional
>> paging).
>>
>> That is, grab the lock when finding a rmap with more than one spte in
>> it (and then keep it locked until the end).
> 
> Hmmï that isn't straightforward and more complex than the approach
> in this patchset. Also it can drop the improvement for shadow mmu that
> gets great improvement by this patchset.
> 
>>
>> For example, nothing prevents lockless walker to move into some
>> parent_ptes chain, right?
> 
> No.
> 
> The nulls can help us to detect this case, for parent_ptes, the nulls points
> to "shadow page" but for rmaps, the nulls points to slot.arch.rmap. There
> is no chance that the ïrmap" is used as shadow page when slot-lock is held.
> 
>>
>> 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,
> itïs 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.


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