linux-kernel - Re: kexec: Clearing registers just before jumping into purgatory

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Message-ID: <20131011165351.GC2772@redhat.com>
Date:	Fri, 11 Oct 2013 12:53:51 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Matthew Garrett <mjg59@...f.ucam.org>
Cc:	Richard Weinberger <richard.weinberger@...il.com>,
	Daniel Kiper <daniel.kiper@...cle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, hbabu@...ibm.com,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Kees Cook <keescook@...omium.org>, kexec@...ts.infradead.org,
	LKML <linux-kernel@...r.kernel.org>, david.vrabel@...rix.com,
	jbeulich@...e.com, keir@....org, xen-devel@...ts.xen.org
Subject: Re: kexec: Clearing registers just before jumping into purgatory

On Fri, Oct 11, 2013 at 05:44:00PM +0100, Matthew Garrett wrote:
> On Fri, Oct 11, 2013 at 06:42:36PM +0200, Richard Weinberger wrote:
> > On Fri, Oct 11, 2013 at 6:39 PM, Matthew Garrett <mjg59@...f.ucam.org> wrote:
> > > On Fri, Oct 11, 2013 at 06:33:23PM +0200, Richard Weinberger wrote:
> > >> On Fri, Oct 11, 2013 at 5:48 PM, Matthew Garrett <mjg59@...f.ucam.org> wrote:
> > >> > On Fri, Oct 11, 2013 at 11:44:50AM -0400, Vivek Goyal wrote:
> > >> >
> > >> >> Just Curious. How is it useful. IOW, what's your use case of booting a new
> > >> >> kernel and then jumping back.
> > >> >
> > >> > I'm kexecing into a kernel with a modified /dev/mem, modifying the
> > >> > original kernel and then jumping back into it.
> > >>
> > >> How do you update the original kernel?
> > >
> > > It's still in RAM, so the same way you'd modify any other arbitrary
> > > physical address?
> > 
> > So, you have a tool like ksplice which patches the kernel in RAM?
> 
> I have /dev/mem and a list of addresses I want to modify.

Why to boot in a second kernel to modify first kernel's RAM. Why not
do it directly from the first kernel itself (until and unless we want
first kernel to be stopped while doing those modifications).

I guess one could hibernate, modify the image and resume to do the
similar thing.

Thanks
Vivek
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