Re: [PATCH 5/5] KVM: paravirt time source

Dong, Eddie
2007-06-21T07:04:40Z

>
>
>I've updated this patch and switched to using a scale/shift
>like Xen is
>doing, but I must admit, I don't understand how it helps adjtime. I
>poked around a bit and it wasn't obvious.

I think the reason is that Xen can't use FP to avoid FP save/restore
at VM Exit time, while PV guest can use FP if want.

No matter scale/shift or FP, hypervisor need to guarantee the time
is accurate enough for years and no overflow. A server may
run for couple years:-)

>
>Why is having {mult=1<<22, shift=22} better for adjtime than {mult=1,
>shift=0}?
>

To avoid overflow after long run. (at least Xen/IA64 is for that
purpose).

thx,eddie