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

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> >
> > Well obviously firmware can do it pre-boot, but I don't know
> > what the impact of the mechanism is on running cpu's.  I'd assume
> > that at least all guest activity would have to be stopped for
> > some not-so-short period (~10-100msec?)
> 
> It depends how physical CPU hotplug is implemented doesn't it. I expect
> there's sufficient firmware involved in such an operation that TSCs
> could
> get synced up before host software gets a look in. I don't think we can
> comment on whether or not there is an issue here without more
> information.
> Also, one reason Intel pushed the CPU hotplug logic is for RAS, and
> offlining CPUs that throw errors, which can clearly be supported with
> no concerns over TSC sync.

OK, then would you accept a patch that disables physical cpu-hot-add
(but not delete) unless enabled with a boot option, if the patch
includes sufficient commenting and dmesg to explain the ramifications?
Then in future if it turns out that TSC syncing is mostly always
handled by firmware (doubtful but possible), the default for the
boot option can be reversed.

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