linux-kernel - Re: [PATCH/RFC] ppc64 livepatch: frameless klp_return

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Date:	Thu, 24 Mar 2016 09:04:49 +0100
From:	Torsten Duwe <torsten@....de>
To:	Balbir Singh <bsingharora@...il.com>
Cc:	Michael Ellerman <mpe@...erman.id.au>,
	Petr Mladek <pmladek@...e.com>, jeyu@...hat.com,
	jkosina@...e.cz, jikos@...nel.org, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, kamalesh@...ux.vnet.ibm.com,
	linuxppc-dev@...abs.org, live-patching@...r.kernel.org,
	mbenes@...e.cz
Subject: Re: [PATCH/RFC] ppc64 livepatch: frameless klp_return_helper using
	odd TOC

On Thu, Mar 24, 2016 at 01:23:01PM +1100, Balbir Singh wrote:
> On 24/03/16 02:58, Torsten Duwe wrote:
> >
> > 1. Heuristics are bad. The better they are, the more subtly the
> >    way they might fail.
[...]
> I missed this yesterday, not on cc, but caught it on the list today

I replied to Michael's last post and removed the back reference to start
a new thread. The list is rather long... sorry I didn't notice.

> Thanks for working on this. I did a quick look, so the CR+4 code

Yes. It's only a proof of concept. That idea is yours, no doubt, and
should be mentioned in the final submission. I already wrote it in my
previous version, where I also have changed the arithmetic to produce
small positive deltas.

> plus heuristics for global/local call detection?

Nope, definitely not! I flag global entries unambiguously.
I had a version with R12/LR heuristics on Monday which I dumped.

>  I'll review this soon - hopefully tonight, but we have a long weekend coming up, so there might be delays. In the meanwhile feel free to add my signed-off-by for the CR+4 code. I am also looking at a different approach -- per thread lr0 stack.

This CR+4 code leaves only 1 slot, so sibling calls are extremely dangerous,
as I mentioned. But with a little attention, this patch works very well.

I mostly wanted to hear opinions about a transient odd TOC value
before I start polishing.

	Torsten