[RFC PATCH v2 0/5] Improvements to console timestamps

[Andrew Cooper <andrew.cooper3@citrix.com>]

This series aims to improve on the current implementation of console
timestamps in Xen.

Patch 1 is a plain optimisation fix and logically independent from the rest of
the series.

Patch 2 changes Xen's idea of when time starts, from when the BSPs TSC was 0,
to when Xen boots.

Patch 3 guesses at AP time calibration earlier during boot, so printk()s using
the new console timestamp have a real stamp, rather than 0s.

Patch 4 is the meat of the series, adding a new timestamp implementation to
printk_start_of_line().

Patch 5 comes as a intermediate suggestion, to retain the old timestamp style,
but to display milliseconds as well.

There is still one bug to fix; The time step when the platform timer start:

    (XEN) [    1.069271] ENABLING IO-APIC IRQs
    (XEN) [    1.073308]  -> Using new ACK method
    (XEN) [    1.077771] ..TIMER: vector=0xF0 apic1=0 pin1=2 apic2=-1 pin2=-1
    (XEN) [    0.017017] Platform timer is 14.318MHz HPET
    (XEN) [    0.021701] Allocated console ring of 16 KiB.

Also, from discussion in the office, it has been suggested that the timestamp
mode/format would be better as build-time configuration rather than boot-time
configuration, and I would have to lean towards agreeing with this.

Furthermore, 3 different timestamp modes would seem to be overkill.

Comments welcome,

~Andrew