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    SubjectRe: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler
    On 07/11/2012 01:17 PM, Christian Borntraeger wrote:
    > On 11/07/12 11:06, Avi Kivity wrote:
    > [...]
    >>> Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for spinlocks, though.
    >> Perhaps x86 should copy this.
    > See arch/s390/lib/spinlock.c
    > The basic idea is using several heuristics:
    > - loop for a given amount of loops
    > - check if the lock holder is currently scheduled by the hypervisor
    > (smp_vcpu_scheduled, which uses the sigp sense running instruction)
    > Dont know if such thing is available for x86. It must be a lot cheaper
    > than a guest exit to be useful

    We could make it available via shared memory, updated using preempt
    notifiers. Of course piling on more pv makes this less attractive.

    > - if lock holder is not running and we looped for a while do a directed
    > yield to that cpu.
    >>> So there is no win here, but there are other cases were diag44 is used, e.g. cpu_relax.
    >>> I have to double check with others, if these cases are critical, but for now, it seems
    >>> that your dummy implementation for s390 is just fine. After all it is a no-op until
    >>> we implement something.
    >> Does the data structure make sense for you? If so we can move it to
    >> common code (and manage it in kvm_vcpu_on_spin()). We can guard it with
    >> CONFIG_KVM_HAVE_CPU_RELAX_INTERCEPT or something, so other archs don't
    >> have to pay anything.
    > Ignoring the name,

    What name would you suggest?

    > yes the data structure itself seems based on the algorithm
    > and not on arch specific things. That should work. If we move that to common
    > code then s390 will use that scheme automatically for the cases were we call
    > kvm_vcpu_on_spin(). All others archs as well.

    ARM doesn't have an instruction for cpu_relax(), so it can't intercept
    it. Given ppc's dislike of overcommit, and the way it implements
    cpu_relax() by adjusting hw thread priority, I'm guessing it doesn't
    intercept those either, but I'm copying the ppc people in case I'm
    wrong. So it's s390 and x86.

    > So this would probably improve guests that uses cpu_relax, for example
    > stop_machine_run. I have no measurements, though.

    smp_call_function() too (though that can be converted to directed yield
    too). It seems worthwhile.

    error compiling committee.c: too many arguments to function

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