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

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Message-ID: <877gdkce6s.fsf@tw-ebiederman.twitter.com>
Date:	Fri, 11 Oct 2013 03:08:43 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Daniel Kiper <daniel.kiper@...cle.com>
Cc:	hbabu@...ibm.com, hpa@...ux.intel.com, keescook@...omium.org,
	vgoyal@...hat.com, kexec@...ts.infradead.org,
	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

Daniel Kiper <daniel.kiper@...cle.com> writes:

> Hi,
>
> Could you explain why do you clear all registers just before jumping
> into purgatory (please look into arch/x86/kernel/relocate_kernel_64.S
> for more details)? There is no any single word about that. I do not
> count comment which states what is going on. purgatory on entry does
> not assume any value in registers. Are you going to use that feature
> for something in the future (e.g. to differentiate between callers
> and/or Linux versions if it be needed)?

It has been a long time now, but as I recall the reason was to just
have things well defined and to make certain that we were not
accidentially exporting anything except the stack pointer for
applications to depend upon.

0/NULL is a good choice because if you are expecting pointer for some
strange reason interesting things happen.

purgatory is definitely not the only target and the C version of
purgatory was actually written well after kexec came into existence.

Is there any particular reason why you are asking?

> By the way, interestingly it is not done if preserve_context is in
> force.

Something different is done, and all of the registers should be
preserved from the when the return to Linux.

In theory you can swap between to kernels with the preserve_context
case.  Technically I like the ability but I don't know that it has ever
achieved much uptake.

Eric

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