RE: [Xen-devel] [RFC] Physical hot-add cpus and TSC

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> >From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> >
> >I implemented this as xen-unstable:21468. This represents a strict
> >improvement on what was in xen-unstable before that (no tsc sync at
> all,
> >ever, because I deleted it about a week ago). Open to further
> improvements,
> >if we can get consensus.
> 
> Thanks for the patch. I will get the system that support CPU hotplug
> tomorrow morning, I can try it then.
> 
> --jyh

Hmmm... I'd be very surprised if this works ever, let alone
always across all hardware.  By "work", I mean that it will
result in an "undetectably small difference" (less than a
cache bounce), as defined and measured by the tsc warp test.
Why?  Because rdtsc is a long instruction (30-100 cycles)
and I'll bet writing to the TSC is even longer.  Plus,
there's a cache bounce overhead added in due to the
synchronization via the in-memory tsc_count variable.

Our firmware guys say that TSC synchronization can't be
implemented algorithmically in firmware... it requires
a simultaneous "reset" signal to all sockets/cores, which
is obviously not an option here.

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