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RE: [Xen-devel] [PATCH] use per-cpu variables in cpufreq

> From: Keir Fraser
> Sent: Monday, May 30, 2011 5:45 PM
> On 30/05/2011 06:47, "Juergen Gross" <juergen.gross@xxxxxxxxxxxxxx> wrote:
> >> Specifically, my fear is that this data gets pushed into the hypervisor
> >> once-only during dom0 boot (via XENPF_set_processor_pminfo). If it is freed
> >> during processor offline, we lose it forever and have no power management
> >> when/if a CPU is brought back online. Worse I suspect your patch as it is
> >> will crash if some CPUs are offline during boot as you'll deference their
> >> per_cpu area which doesn't actually exist unless a CPU is online. You can
> >> test this for yourself by adding a maxcpus=1 boot parameter for Xen.
> >
> > Indeed.
> >
> > Just to understand this completely:
> > So when is this info set up for hot-plugged cpus? And what happens if
> > a cpu module is removed and later replaced by another module with
> > more cores (or threads) than the original one?
> > I think the processor pminfo should be set in this case during the hotplug
> > handling.
> Well, there is a difference between logical and physical cpu hotplug. Xen is
> capable of bringing CPUs online and offline without them actually being
> physically plugged/unplugged from the mainboard. Indeed our physical hotplug
> support is relatively new and I would suspect not much used (and it supports
> only physical insertion, not removal!).
> Frankly there are a number of questions around CPUs that are physically
> plugged in after boot:
>  * How does per-CPU ACPI state like PM info get set up?

A hotplug-able cpu still has an ACPI CPU object in DSDT table, which may exist
in original DSDT table, or dynamically loaded later upon hotplug event. Once
dom0 ACPI recognizes a new CPU object, it will notify Xen about discovered
pm information.

>  * In a system where TSCs are otherwise all perfectly in sync, does the
> firmware help us by setting up the new CPUs' TSCs likewise?

Here what firmware can do is similar to what Xen can do, which can't ensure
you a truly synchronized TSC on new CPU.


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