Re: [RFC PATCH 2/2] KVM: selective write protection using dirty bitmap

[Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>]

(2010/11/18 22:06), Avi Kivity wrote:
> On 11/18/2010 07:15 AM, Takuya Yoshikawa wrote:

>>
>> We can also use this to selectively write protect pages to reduce unwanted page
>> faults in the future.
>>
>
> Looks like a good approach. Any measurements?
>

OK, I'll do some tests to select a right approach.

Personally, and some as a job if possible, I'm planning various things.


a) Frame buffer: small bitmap size (my hobby now)

    Performance:
      Both function level and guest side measurements is necessary. *1
      For interactivity, qemu side intelligence may be needed.
      Note: qemu side dirty bitmap optimization by Kemari team is pending now.

    Correctness:
      I doubt qemu side but I'm not good at device level VGA details yet, sorry.

    *1) I once tried to update dirty bitmap without srcu sync by using
        atomic_clear_mask() word by word. This seemed to be working and
        get_dirty_log() was not bad. But when I tested x11perf inside the guest,
        the numbers were worse than the original implementation (LOCK_PREFIX
        might do bad things to the cache and srcu reader side suffered, I guess).

        Good SRCU!


b) Live-migration: large bitmap size (my target is ~4GB RAM now)

    Performance:
      Scalability is important in both RAM size and number of dirty pages point
      of view. We need to achieve this goal without sacrificing the low workload
      case.

    Correctness:
      I'm trying to make live-migration fail in the condition of without mst's fix
      by multi threaded high work load. SMP is necessary.


>
>> +
>> + if (!(gfn_offset || (gfn % huge)))
>> + break;
>
> Why?

I preserved these lines from Lai's original.
 >> http://www.spinics.net/lists/kvm/msg35871.html

But OK, I found some duplication probably. I'll fix.


>
> wrt. O(1) write protection: hard to tell if the two methods can coexist. For direct mapped shadow pages (i.e. ep/npt) I think we can use the mask to speed up clearing of an individual sp's sptes.
>

OK, I'll test both with and without NPT.
The result may lead us to the right direction.



Takuya