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    SubjectRe: virtio: Add memory statistics reporting to the balloon driver
    Rusty Russell wrote:
    > On Wed, 11 Nov 2009 08:22:42 am Anthony Liguori wrote:
    >> Rusty Russell wrote:
    >>> On Tue, 10 Nov 2009 03:02:06 am Adam Litke wrote:
    >>>> A simpler approach is to collect memory statistics in the virtio
    >>>> balloon driver and communicate them to the host via the device config space.
    >>> There are two issues I see with this. First, there's an atomicity problem
    >>> since you can't tell when the stats are consistent. Second, polling is
    >>> ugly.
    >>> A stats vq might solve this more cleanly?
    >> This turns out to not work so nicely. You really need bidirectional
    >> communication. You need to request that stats be collected and then you
    >> need to tell the hypervisor about the stats that were collected. You
    >> don't need any real correlation between requests and stat reports either.
    > You register an outbuf at initialization time. The host hands it back when
    > it wants you to refill it with stats.

    That's strangely backwards. Guest send a stat buffer that's filled out,
    host acks it when it wants another. That doesn't seem bizarre to you?

    >> This really models how target/actual work and I think it suggests that
    >> we want to reuse that mechanism for the stats too.
    > Sure, I want to. You want to. It's simple.
    > But the universe is remarkably indifferent to what we want. Is it actually
    > sufficient or are we going to regret our laziness?

    It's not laziness, it's consistency. How is actual different than free
    memory or any other stat?

    > Cheers,
    > Rusty.


    Anthony Liguori

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