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

Xiao Guangrong
2013-11-26T03:10:31Z

On 11/25/2013 10:23 PM, Marcelo Tosatti wrote:
> On Mon, Nov 25, 2013 at 02:48:37PM +0200, Avi Kivity wrote:
>> On Mon, Nov 25, 2013 at 8:11 AM, Xiao Guangrong
>> <xiaoguangrong@linux.vnet.ibm.com> wrote:
>>>
>>> On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote:
>>
>> <snip complicated stuff about parent_pte>
>>
>> I'm not really following, but note that parent_pte predates EPT (and
>> the use of rcu in kvm), so all the complexity that is the result of
>> trying to pack as many list entries into a cache line can be dropped.
>> Most setups now would have exactly one list entry, which is handled
>> specially antyway.
>>
>> Alternatively, the trick of storing multiple entries in one list entry
>> can be moved to generic code, it may be useful to others.
>
> Yes, can the lockless list walking code be transformed into generic
> single-linked list walking? So the correctness can be verified
> independently, and KVM becomes a simple user of that interface.

I'am afraid the signle-entry list is not so good as we expected. In my
experience, there're too many entries on rmap, more than 300 sometimes.
(consider a case that a lib shared by all processes).

>
> The simpler version is to maintain lockless walk on depth-1 rmap entries
> (and grab the lock once depth-2 entry is found).

I still think rmap-lockless is more graceful: soft mmu can get benefit
from it also it is promising to be used in some mmu-notify functions. :)