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    Subject[PATCH 06/58] KVM: Reduce misfirings of the fork detector
    The kvm mmu tries to detects forks by looking for repeated writes to a
    page table. If it sees a fork, it unshadows the page table so the page
    table copying can proceed at native speed instead of being emulated.

    However, the detector also triggered on simple demand paging access patterns:
    a linear walk of memory would of course cause repeated writes to the same
    pagetable page, causing it to unshadow prematurely.

    Fix by resetting the fork detector if we detect a demand fault.

    Signed-off-by: Avi Kivity <>
    drivers/kvm/paging_tmpl.h | 4 ++++
    1 files changed, 4 insertions(+), 0 deletions(-)

    diff --git a/drivers/kvm/paging_tmpl.h b/drivers/kvm/paging_tmpl.h
    index 73ffbff..bc64cce 100644
    --- a/drivers/kvm/paging_tmpl.h
    +++ b/drivers/kvm/paging_tmpl.h
    @@ -421,6 +421,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr,
    pgprintk("%s: guest page fault\n", __FUNCTION__);
    inject_page_fault(vcpu, addr, walker.error_code);
    + vcpu->last_pt_write_count = 0; /* reset fork detector */
    return 0;

    @@ -442,6 +443,9 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr,


    + if (!write_pt)
    + vcpu->last_pt_write_count = 0; /* reset fork detector */
    * mmio: emulate if accessible, otherwise its a guest fault.
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      Last update: 2007-06-17 11:49    [W:0.020 / U:79.016 seconds]
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