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    SubjectRe: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault
    On 04/29/2012 04:50 PM, Takuya Yoshikawa wrote:

    > On Fri, 27 Apr 2012 11:52:13 -0300
    > Marcelo Tosatti <> wrote:
    >> Yes but the objective you are aiming for is to read and write sptes
    >> without mmu_lock. That is, i am not talking about this patch.
    >> Please read carefully the two examples i gave (separated by "example)").
    > The real objective is not still clear.
    > The ~10% improvement reported before was on macro benchmarks during live
    > migration. At least, that optimization was the initial objective.
    > But at some point, the objective suddenly changed to "lock-less" without
    > understanding what introduced the original improvement.
    > 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.

    > If the path being introduced by this patch is really fast, isn't it
    > possible to achieve the same improvement still using mmu_lock?
    > Note: During live migration, the fact that the guest gets faulted is
    > itself a limitation. We could easily see noticeable slowdown of a
    > program even if it runs only between two GET_DIRTY_LOGs.

    Obviously no.

    It depends on what the guest is doing, from my autotest test, it very
    easily to see that, the huge improvement is on bench-migration not

    >> The rules for code under mmu_lock should be:
    >> 1) Spte updates under mmu lock must always be atomic and
    >> with locked instructions.
    >> 2) Spte values must be read once, and appropriate action
    >> must be taken when writing them back in case their value
    >> has changed (remote TLB flush might be required).
    > 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?

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      Last update: 2012-05-02 08:21    [W:2.391 / U:0.144 seconds]
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