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Re: [Xen-devel] [PATCH 2/2] RFC: Xen pad logic

On Mon, Mar 26, 2012 at 06:18:36AM +0000, Liu, Jinsong wrote:
> Konrad Rzeszutek Wilk wrote:
> > On Thu, Feb 23, 2012 at 01:31:25PM +0000, Liu, Jinsong wrote:
> >>> From ba9abf6ee7e5fe0515e2d51b14743c8d5416285c Mon Sep 17 00:00:00
> >>> 2001 
> >> From: Liu, Jinsong <jinsong.liu@xxxxxxxxx>
> >> Date: Fri, 24 Feb 2012 02:18:02 +0800
> >> Subject: [PATCH 2/2] Xen pad logic
> >> 
> >> This patch implement Xen pad logic, and when getting pad device
> >> notification, it hypercalls to Xen hypervisor for core parking.
> > 
> > Can you explain to me how and what pad device is? And how it functions
> > right now in baremetal? And what kind of hardware do you need to use
> > this? 
> > And what happens if you do not use it? Can one ignore the "pad"
> > support? 
> > Please assume that I've a basic understanding of ACPI.
> > 
> > 
> > Also, what happens now, if the this patch is not implemented? What
> > will/is dom0 doing without these patches (so 3.2 for example on this
> > machine)? 
> > Is it just idling using mwait on idle CPUs and ending up trapping in
> > the hypervisor? Or is not mwaiting since the cstate.c doesn't get
> > executed since we have: 
> > 
> >        boot_option_idle_override = IDLE_HALT;
> > 
> > in arch/x86/xen/setup.c ?
> Pad is an ACPI device used to direct os taking some action (depend on os 
> itsef) for the sake of power consumption. 2 objs (PUR and OST) could be 
> declared under PAD and as bios/os interface.
> PAD itself is pointless, unless it co-work with some external power control 
> s/w (like NPTM). For example currently in baremetal, NPTM engine trigger sci 
> to ospm, then evaluate and call sci handler, through which bios notify os 
> with PUR value by which os could take corresponding action and feedback bios 
> through OST. I don't think it's a problem if user don't use PAD or ignore it, 
> it only make some external power control s/w unuseable.

What is NPTM? Sounds like a SMI firmware?

> As for how os handle pad notify is os business. Native kernel use round robin 
> and mwait, considering some app workload affinity (I was told so). For xen we 
> don't need care it since it's a question of vcpu level. For xen acpi_pad, 
> dom0 patches used to parse PUR and hypercall to hypervisor, which in turn 
> idle pcpu by its own algorithm.

I presume you have some of this hardware - if you launch the latest 
linus/master (along with these
patches https://bugzilla.redhat.com/show_bug.cgi?id=804347 patches) and compile 
and are running under Xen 4.1, what happens when the _PUR notifcation takes 

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