linux-kernel - Re: [Xen-devel] [PATCH 3/8] ACPI: processor: add __acpi_processor_[un]register

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Date:	Thu, 22 Dec 2011 23:01:03 -0400
From:	Konrad Rzeszutek Wilk <konrad@...nok.org>
To:	"Tian, Kevin" <kevin.tian@...el.com>
Cc:	"jeremy@...p.org" <jeremy@...p.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"Ian.Campbell@...rix.com" <Ian.Campbell@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"mike.mcclurg@...rix.com" <mike.mcclurg@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stefan.bader@...onical.com" <stefan.bader@...onical.com>,
	"rjw@...k.pl" <rjw@...k.pl>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"liang.tang@...cle.com" <liang.tang@...cle.com>,
	"Yu, Ke" <ke.yu@...el.com>,
	"konrad@...nel.org" <konrad@...nel.org>,
	"lenb@...nel.org" <lenb@...nel.org>
Subject: Re: [Xen-devel] [PATCH 3/8] ACPI: processor: add __acpi_processor_[un]register_driver helpers.

> > OK. Lets put the # VCPU != PCPU aside. Say dom0 will boot with all
> > CPUs and then later on the admin starts unplugging them.
> 
> This should be communicated to major Xen based distributions, so that it's
> an agreed approach since in majority case dom0 is configured as UP or
> a few VCPUs.

I am not saying that is it the agreed approach. There has to be
flexibility in supporting both. But what I want to understand whether
the requirement for VCPU != PCPU can be put aside and put in the drivers
later on.

So that the first approach is not changing the generic drivers (much).
The reason I am asking about this is two-fold:
 1). For new distros (Ubuntu, Fedora), the default is all VCPUs.
     Enterprising users might use dom0_max_vcpus to limit the VCPU count,
     but most won't.
     Which mean we can concentrate on bringing the _Pxx/_Cxx parsing
     up to the hypervisor. Which is really neccessary on any chipset
     which has the notion of TurboBoost (otherwise the Xen scheduler
     won't pick this up and won't engage this mode in certain
     workloads).
 2). The ACPI maintainers are busy with ACPI 5.0. I don't know how
     much work this is, but it probably means tons of stuff with
     embedded platforms and tons of regression testing. So if there
     is a patch that does not impact the generic code much (or any)
     it will make their life easier. Which also means we can built
     on top that for the VCPU != PCPU case.

That is what I am trying to understand.
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