[Xen-devel] query related to NOW()

["Pradeep Singh, TLS-Chennai" <pradeep_s@xxxxxx>]

Hi All,

NOW() macro which is used to get current time on xen, makes use of rdtscll().
rdtscll reads the 64 bit timer value from the TSC register on x86 architecture.
AFAIK TSC registers are no longer considered a safe guarntee to determine exact timings on SMP, multicore platform as well as may give incorrect values when used after a wake up from hibernation.

My query is then why is TSC register used in xen to get the current time??
Does this means Xen doesnot have good SMP support and also doesnot support hibernation?
Just to add using TSC register means hampering portability, right? Then why does Xen uses it?

Any pointers or links please.

Please CMIIW.

Thank you.

Pradeep Singh

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