Re: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 07/24/09 01:04, Keir Fraser wrote:
> Okay, so the issue you are worried about is not specific to Xen. So how is
> native Linux tackling this, for example?
>   

Linux will use the tsc where possible, but regularly assesses its
perceived accuracy and will move to a different clocksource if the tsc
appears to the playing up.  I don't think it ever assumes the tsc is
synced between CPU/cores.

It allows rdtsc from usermode, but it is generally considered to be very
buggy and ill-defined behaviour.  It makes no attempt to make usermode
rdtsc in any way meaningful.  The exception is the vgettimeofday
vsyscall which does Xen-like timekeeping, in which it gets the tsc,cpu
tuple atomically, then scales it with timing parameters from the kernel.

    J

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