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    SubjectRe: [PATCH 00/13] KVM: MMU: fast page fault
    On Tue, 10 Apr 2012 13:39:14 +0300
    Avi Kivity <avi@redhat.com> wrote:

    > On 04/09/2012 10:46 PM, Marcelo Tosatti wrote:
    > > Perhaps the mmu_lock hold times by get_dirty are a large component here?
    > > If that can be alleviated, not only RO->RW faults benefit.
    > >
    > >
    >
    > Currently the longest holder in normal use is probably reading the dirty
    > log and write protecting the shadow page tables.
    >
    > We could fix that by switching to O(1) write protection
    > (write-protecting PML4Es instead of PTEs). It would be interesting to
    > combine O(1) write protection with lockless write-enabling.
    >

    As Marcelo suggested during reviewing srcu-less dirty logging, we can
    mitigate the get_dirty's mmu_lock hold time problem cleanly, locally in
    get_dirty_log(), by using cond_resched_lock() -- although we need to
    introduce cond_rescheck_lock_cb() to conditionally flush TLB.

    I have already started that work.

    Actually I introduced rmap based get_dirty for that kind of fine-grained
    contention control.

    I think we should do our best not to affect mmu so much just for the
    limited time of live migration.

    Takuya


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