Collecting entropy from device_attach() times.

[Ben Laurie]

On Thu, Sep 27, 2012 at 10:49 AM, Dag-Erling Smørgrav <des at des.no> wrote:
> RW <rwmaillists at googlemail.com> writes:
>> "Dag-Erling Smørgrav" <des at des.no> writes:
>> > You can't rely on the existence of a TSC.  I would suggest using the
>> > fractional part of binuptime instead.
>> get_cyclecount() is supposed to be platform independent and should
>> fall-back to nanotime(9) if TSC or equivalent is absent.
>
> I just thought of another issue with get_cyclecount().
>
> On machines with TSCs, its resolution varies with the CPU's speed
> (nominal or actual, depending on the exact model).  This means that
> attachtime measurements have far lower resolution and therefore less
> entropy on slow machines than on fast ones.
>
> This doesn't mean we can't use get_cyclecount(), just that we shouldn't
> base our entropy estimates on data gathered on a fast system.

We should certainly see how things look on slow systems, but note that
if the resolution is lower, then the measurements will also be smaller
(assuming attachment takes similar time), and so we will claim less
entropy anyway :-)