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

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Date:   Wed, 22 Jul 2020 15:51:39 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Joe Lawrence <joe.lawrence@...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 Wed, Jul 22, 2020 at 01:03:03PM -0400, Joe Lawrence wrote:
> 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.

It's an incremental step towards _something_.  Whether that's a cohesive
patch creation guide, or just a growing hodgepodge of random documents,
it may be too early to say :-)

> > 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. :)

Are experienced livepatch developers the intended audience?  If so I
question what value this document has in its current form.  Presumably
experienced livepatch developers would already know this stuff.

> > 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.

I wouldn't say it *needs* to have that context.  But it would be a lot
more useful with it.  As you pointed out, the existing documents do need
to be reorganized into a more cohesive whole.

-- 
Josh