linux-kernel - RE: [V4 PATCH 4/4] x86/apic: Introduce noextnmi boot option

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Date:	Tue, 13 Oct 2015 11:55:10 +0000
From:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>
To:	"'Borislav Petkov'" <bp@...en8.de>
CC:	"'Peter Zijlstra'" <peterz@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Ingo Molnar <mingo@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vivek Goyal <vgoyal@...hat.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>
Subject: RE: [V4 PATCH 4/4] x86/apic: Introduce noextnmi boot option

Hello, Boris

Sorry for the late reply.

> On Mon, Oct 05, 2015 at 09:21:02AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > So, the problem for you is that "noextnmi" option is visible and effective
> > in the first kernel, isn't it?
> 
> No, such an option shouldn't exist at all. You should be passing
> information *in* *a* *different* *manner* to the kdump kernel - not with
> a kernel command line option.

Sorry, I couldn't find out the reason why I shouldn't use cmdline option.
It doesn't need new user I/F to inform the 1st kernel about masking/unmasking
external NMI in the 2nd kernel, doesn't need new data passing infrastructure,
and is easy to configure that.  Also, "elfcorehdr" and "disable_cpu_apicid"
have already been introduced as cmdline options for dump capture kernel.
This means the cmdline option approach would be mostly acceptable.

> I get the feeling I'm starting to sound like a broken record on this
> mail thread... :-(
> 
> One other thing we could probably try to do is use boot_params which
> is, IIUC, passed to the second kernel. So we can add another bit to
> boot_params.hdr.loadflags or so and use that. Or something similar.

I think using boot_params would be worse than ELF header approach.
It needs to reserve boot_params bits for all boot loaders.

Regards,

Hidehiro Kawai
Hitachi, Ltd. Research & Development Group