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    SubjectRe: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault
    On 05/03/2012 08:15 AM, Takuya Yoshikawa wrote:

    > On Wed, 02 May 2012 13:39:51 +0800
    > Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> wrote:
    >
    >>> Was the problem really mmu_lock contention?
    >
    >> Takuya, i am so tired to argue the advantage of lockless write-protect
    >> and lockless O(1) dirty-log again and again.
    >
    > You are missing my point. Please do not take my comments as an objection
    > to your whole work: whey do you feel so?
    >


    Takuya, i am sorry, please forgive my rudeness! Since my English is
    so poor that it is easy for me to misunderstand the mail. :(

    > I thought that your new fast-page-fault path was fast and optimized
    > the guest during dirty logging.
    >
    > So in this v4, you might get a similar result even before dropping
    > mmu_lock, without 07/10?, if the problem Marcelo explained was not there.
    >


    Actually, the improvement is larger than v2/v3 if ept is enabled, but
    it is lower for ept disabled. This is because the fask-fask (rmap.WRITABLE bit)
    is dropped for better review.

    >
    > Of course there is a problem of mmu_lock contention. What I am suggesting
    > is to split that problem and do measurement separately so that part of
    > your work can be merged soon.
    >
    > Your guest size and workload was small to make get_dirty hold mmu_lock
    > long time. If you want to appeal the real value of lock-less, you need to
    > do another measurment.
    >
    >
    > But this is your work and it's up to you. Although I was thinking to help
    > your measurement, I cannot do that knowing the fact that you would not
    > welcome my help.
    >


    Of course, any measurement is appreciative!

    >
    >>> Although I am not certain about what will be really needed in the
    >>> final form, if this kind of maybe-needed-overhead is going to be
    >>> added little by little, I worry about possible regression.
    >
    >> Well, will you suggest Linus to reject all patches and stop
    >> all discussion for the "possible regression" reason?
    >
    > My concern was for Marcelo's examples, not your current implementation.
    > If you can show explicitely what will be needed in the final form,
    > I do not have any concern.
    >
    >
    > Sorry for disturbing.


    Sorry again.

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