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Re: [Xen-devel] [PATCH 0/2] Improve hpet accuracy

On 6/6/08 21:29, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Although hwhpet=1 is a fine alternative in many cases, it may
> be unavailable on some systems and may cause significant performance
> issues on others.  So I think we will still need to track down
> the poor accuracy when hwhpet=0.  And if for some reason
> Xen system time can't be made accurate enough (< 0.05%), then
> I think we should consider building Xen system time itself on
> top of hardware hpet instead of TSC... at least when Xen discovers
> a capable hpet.

Yes, this would be a sensible extra timer_mode: have hvm_get_guest_time()
call to the platform time read function, and bypass TSC altogether. This
would be cleaner than having only the vHPET code punch through to the
physical HPET: instead we have the boot-time chosen platform timesource used
by all virtual timers.

> Or maybe there's a computation error somewhere in the hvm hpet
> scaling code?  Hmmm...

Possibly there are bugs in the hpet device model which are fixed by Dave's
patch. If this is actually the case, it would be nice to break those out as
separate patches, as I think an 11% drift must largely be due to
device-model bugs rather than relatively insignificant differences between
hvm_get_guest_time() and physical HPET main counter.

 -- Keir

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