linux-kernel - Re: [PATCH 1/2] docs/livepatch: Add new compiler considerations doc

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Date:   Wed, 22 Jul 2020 13:03:03 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] docs/livepatch: Add new compiler considerations doc

On 7/21/20 7:04 PM, Josh Poimboeuf wrote:
> On Tue, Jul 21, 2020 at 12:14:06PM -0400, Joe Lawrence wrote:
>> Compiler optimizations can have serious implications on livepatching.
>> Create a document that outlines common optimization patterns and safe
>> ways to livepatch them.
>>
>> Signed-off-by: Joe Lawrence <joe.lawrence@...hat.com>
> 
> There's a lot of good info here, but I wonder if it should be
> reorganized a bit and instead called "how to create a livepatch module",
> because that's really the point of it all.
> 

That would be nice.  Would you consider a stand-alone 
compiler-optimizations doc an incremental step towards that end?  Note 
that the other files (callbacks, shadow-vars, system-state) in their 
current form might be as confusing to the newbie.

> I'm thinking a newcomer reading this might be lost.  It's not
> necessarily clear that there are currently two completely different
> approaches to creating a livepatch module, each with their own quirks
> and benefits/drawbacks.  There is one mention of a "source-based
> livepatch author" but no explanation of what that means.
> 

Yes, the initial draft was light on source-based patching since I only 
really tinker with it for samples/kselftests.  The doc was the result of 
an experienced livepatch developer and Sunday afternoon w/the compiler. 
I'm sure it reads as such. :)

> Maybe it could begin with an overview of the two approaches, and then
> delve more into the details of each approach, and then delve even more
> into the gory details about compiler optimizations.
> 

Up until now, the livepatch documentation has danced around the 
particular creation method and only described the API in abstract.  If a 
compiler considerations doc needs to have that complete context then I'd 
suggest we reorganize the entire lot as a prerequisite.

> Also the kpatch-build section can reference the patch author guide which
> we have on github.
> 

Good point.  I think there are a few kpatch-specific implications 
(sibling call changes maybe) to consider.

-- Joe