Folks --
My colleague and I are looking at the linux update_wallclock()
algorithm and the same (more or less) algorithm in FreeBSD. And
we''ve noticed an independent_wallclock variable and associated
functions in Linux that look like they provide some utility.
In Linux:
static void update_wallclock(void)
{
shared_info_t *s = HYPERVISOR_shared_info;
do {
shadow_tv_version = s->wc_version;
rmb();
shadow_tv.tv_sec = s->wc_sec;
shadow_tv.tv_nsec = s->wc_nsec;
rmb();
} while ((s->wc_version & 1) | (shadow_tv_version ^ s-
>wc_version));
if (!independent_wallclock)
__update_wallclock(shadow_tv.tv_sec,
shadow_tv.tv_nsec);
}
In FreeBSD there isn''t the check for independent_wallclock, nor any
__update_wallclock()-like functionality. However, it appears that
the __update_wallclock() algorithm is necessary.
My questions are:
1) Is support for independent_wallclock functionality "relatively"
new? (i.e., Last couple of years? Or, since Xen 2.0?)
2) Is (!independent_wallclock) ever true? If so, what sort of
environments? Is it true for dom0? Is there a guest mode where
having an independent wallclock is appropriate?
3) Is most everything in the linux-xen-sparse tree under the LGPL
license?
We''re working to port the code from linux-xen-sparse/time-xen.c to
FreeBSD in order to fix our problem; we''re just curious about the
environment in which the code runs.
-- Randy
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