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    SubjectRe: [PATCH 9/11] Panic delay fix
    Alan wrote:
    >> We'd have to audit and figure out what udelays are for hardware and
    >> which are not, but the evidence is that the vast majority of them are
    >> for hardware and not needed for virtualization.
    > Which is irrelevant since the hardware drivers won't be used in a
    > virtualised environment with any kind of performance optimisation.

    Which is why an audit is irrelevant for the most part. Note on the
    performance below.

    >> Changing udelay to "hardware_udelay" or something all over the kernel
    >> would have delayed the paravirt_ops merge by an infinite amount 8)
    > paravirt_ops has no business fiddling with udelay. Not only does it
    > create more code bloat and stalls in relatively fast paths but its
    > optimising the wrong thing anyway.

    ??? I fail to see the code bloat and also the fast paths. Which fast
    paths use udelay?

    > My performance sucks -> optimise out udelay is the wrong approach. My
    > performance sucks, switch to the virtual block driver is the right
    > approach, and a virtual block driver won't be using udelay anyway

    This is not to stop performance from sucking. It doesn't. This is not
    an "approach". Sure, a virtual block driver won't be using udelay.
    Everyone else who writes hypervisors writes virtual block drivers
    because they don't have optimized I/O emulation for real hardware.
    Their performance sucks without it because they have to go switch to
    some other context and run a device emulator. Our doesn't. We have
    optimized almost every I/O device we emulate. But sitting around
    spinning in udelay is wasting everybody's time. There is an overhead
    cost to trapping out on I/O instructions. Removing the udelays that
    typically happen around I/O instructions causes the emulation to break even.

    And that is a good thing. It's certainly not required, nor is it a
    significant win while the kernel is running. It does cut the boot time
    by a lot, and you will notice an obvious difference with a much faster
    kernel boot simply because a lot of the hardware setup has very
    conservative udelays which take a lot of time during device
    initialization. Since boot time * number of reboots has a direct impact
    on the number of 9's you can claim for uptime, this is actually a large
    win for reliability.

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