Re: Xen system skew MUCH worse than tsc skew (was RE: [Xen-devel] RE: [PATCH] record max stime skew (was RE: [PATCH] strictly increasing hvm guest time))

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 11/7/08 21:53, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> 1) by a boot option, or
> 2) by the TSC_CONSTANT cpu flag, or
> 3) when determined dynamically to be safe using code similar
>    to arch/x86/tsc_sync.c in recent Linux kernels
> 
> (1) is by far the easiest (perhaps not too late for 3.3?)
> while (3) is clearly the best for users but adds lots of
> code (bloat/untested)

(1) is perhaps fine.

How does (2) work? The individual CPUs do not know whether they are
synchronised across the mainboard. I think constant-tsc is necessary
(individual CPUs must not vary their multiplier of the input clock rate) but
may not be sufficient.

I don't know how much code is involved in (3).

 -- Keir



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