[Xen-devel] Re: [PATCH] clocksource=tsc

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 15/7/08 15:44, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Hmmm... One thing I was trying to do with the special
> casing in get_s_time() was to avoid using a dynamically
> changing scaling (t->tsc_scale).  If I'm not mistaken,
> t->tsc_scale is recalculated every EPOCH and thus is
> a potential source of stime jitter.  Seems unnecessary
> just to make the code look cleaner.

My patch disables the per-epoch calibration.

Actually in this mode of operation we hardly need a platform timer *at all*.
The idea is that we let the TSCs free-run, because we know they will behave.
Returning to 32-bit read_counter(), and having NULL read_counter when
clocksource=tsc would be another possibility...

> One other nit:  Especially if the above is changed, do
> you really prefer a strcmp vs a global variable (or
> checking for size == 64) for determining if the tsc
> is the platform timer?

It's hidden in a macro and it's called very rarely. So I think it's fine.

 -- Keir



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