kexec: Clearing registers just before jumping into purgatory

[Daniel Kiper]

On Fri, Oct 11, 2013 at 03:08:43AM -0700, ebiederm at xmission.com wrote:
> Daniel Kiper <daniel.kiper at oracle.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.

This covers more or less with my expectations.

> 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?

Yes, we (Xen guys) are discussing is it worth to do it or not in our
kexec implementation. I think that yes because we used Linux Kernel
kexec implementation as a base for our work and we use kexec-tools too.
So we should be aligined to what currently is in the wild. David do not
agree with me. You could find more here:

http://lists.xen.org/archives/html/xen-devel/2013-10/msg00710.html
http://lists.xen.org/archives/html/xen-devel/2013-10/msg00296.html

What is your opinion in that case?

> > 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.

I expected that but purgatory does nothing with them.
However, maybe I missed something.

> 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.

I think that this is nice idea too. However, I have not seen its usage in real.
Even once there was an idea to remove that stuff from Linux Kernel.

Daniel