Re: [Xen-devel] Kernel printk timestamps and walltime drift

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 13/6/08 23:02, "Roger Cruz" <rcruz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:

> Was I correct in my understanding on how the timestamp is being obtained
> via this call sequence?
> 
> -> vprintk -> printk_clock  -> sched_clock -> rdtscll
> 
> In other words, does your patchset use the rdtscll instruction on an
> i386, pv, 32-bit linux to compute determine that time?  If not, how is
> it derived (what file/function should I look at for sched_clock?).

The guest calculates current Xen system time based on extrapolating from a
Xen-supplied timestamp using current TSC:
 sys_time = sys_timestamp + (RDTSC - tsc_timestamp) * scale_factor

Where three terms on the RHS of the formula are supplied by Xen in the
shared_info page, and the fourth is returned by the RDTSC CPU instruction.

> While you are right that the only artifact that we have observed is the
> drifting timestamps, a future product of ours may need to have an
> accurate TSC presentation to the VM and if the time is being derived
> from the TSC as I'm conjecturing, then this drift is something we're
> going to have to worry about then.

Depends what you mean by accurate. PV guests see the underlying host TSC
directly. In that sense what they see is 100% accurate!

 -- Keir

> Thank you Keir and Dan 



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