linux-kernel - Re: [PATCH] acpi memory hotplug, add parameter to disable memory hotplug for kexec

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Date:	Fri, 10 Jan 2014 02:31:31 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Prarit Bhargava <prarit@...hat.com>, linux-acpi@...r.kernel.org,
	Len Brown <lenb@...nel.org>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Dave Young <dyoung@...hat.com>,
	Kexec Mailing List <kexec@...ts.infradead.org>
Subject: Re: [PATCH] acpi memory hotplug, add parameter to disable memory hotplug for kexec

On Thursday, January 09, 2014 10:00:53 AM Vivek Goyal wrote:
> On Thu, Jan 09, 2014 at 12:00:29AM +0100, Rafael J. Wysocki wrote:
> 
> [..]
> > > The system then panics and the kdump/kexec kernel boots.  During this boot
> > > ACPi is initialized and the kernel (as can be seen above)
> > 
> > Which is a bug.  You're not supposed to initialize ACPI twice in a row.
> 
> [CC lkml, kexec mailing list, dave young]
> 
> It is a fresh instance of kernel booting and it is initializing its data
> structures fresh. It is *not* re-initializing ACPI in same kernel.

I know.  The problem is that the BIOS is generally not stateless and it
remebers stuff.  In particular, there are a few handshakes done during the ACPI
initialization and they cannot be repeated without resetting the BIOS to the
initial state, which basically means a reboot.

That is kind of orthogonal to the problem at hand, though.

> > > This patchset resolves the problem by adding a kernel parameter,
> > > no_memory_hotplug, to disable ACPI memory hotplug.  It can be added by default
> > > as a parameter to the kexec/kdump kernel so the kernel boots correctly.
> > 
> > This problem is specific to kexec/kdump, so please don't add *generic* command
> > line parameters to address this.
> > 
> 
> There are other command line options to solve kdump problems. In general
> one might want to disable memory hogplug on the fly even if it is compiled
> in the kernel. So it can act as a good debugging aid.
> 
> Secondly, it can be specified with memmap=exactmap and mem=X paramters to
> make sure no memory is hot added in the system.
> 
> So I can see other usages of this parameter. To me it makes sense to have
> a separate command line option to disable memory hotplug feature on the
> fly.

Well, if there are other uses, then fine.

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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