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    SubjectRe: [PATCH RFC] virtio: put last seen used index into ring itself
    On Fri, May 07, 2010 at 01:03:28PM +0930, Rusty Russell wrote:
    > On Thu, 6 May 2010 03:49:46 pm Michael S. Tsirkin wrote:
    > > Now, I also added an mb() in guest between read and write so
    > > that last used index write can not get ahead of used index read.
    > > It does feel good to have it there, but I can not say why
    > > it's helpful. Works fine without it, but then these
    > > subtle races might be hard to trigger. What do you think?
    >
    > I couldn't see that in the patch? I don't think it's necessary
    > though, since the write of depends last_used depends on the read of
    > used (and no platform we care about would reorder such a thing).

    Well, there's no data dependency, is there?

    > I'm reasonably happy, but we should write some convenient test for
    > missing interrupts.
    >
    > I'm thinking of a sender which does a loop: blasts 1MB of UDP packets,
    > then prints the time and sleep(1). The receiver would print the time
    > every 1MB of received data. The two times should almost exactly correspond.
    >
    > Assuming that the network doesn't overflow and lose stuff, this should
    > identify any missing wakeup/interrupts (depending on direction used).
    >
    > Cheers,
    > Rusty.


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