Re: [Xen-devel] [RFC v3 07/13] tables.h: add linker table support

["Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>]

On Fri, Aug 12, 2016 at 10:23:34PM +0200, Greg KH wrote:
> On Fri, Aug 12, 2016 at 07:04:52PM +0200, Luis R. Rodriguez wrote:
> > Alright, how's this new description:
> > 
> > diff --git a/init/Kconfig b/init/Kconfig
> > index cac3f096050d..73e4890c24c4 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -53,6 +53,34 @@ config CROSS_COMPILE
> >       need to set this unless you want the configured kernel build
> >       directory to select the cross-compiler automatically.
> >  
> > +config BUILD_AVOID_BITROT
> > +   bool "Always force building specially annotated targets"
> > +   default n
> > +   help
> > +     If enabled then the the special table-* Makefile targets will always
> > +     be forced to be compiled even if their respective CONFIG_ option has
> > +     been disabled, but its objects will only be linked in if the same
> > +     respective CONFIG_ option has been enabled. This helps avoid code
> > +     bit rot issues, use for these targets should be carefully considred
> > +     by maintainers. You can safely enable this option at the expense of
> > +     increasing compile time. Enabling this option helps avoid code bit
> > +     rot by taking advantage of the facilities provided and enabled by
> > +     using linker tables documented under:
> 
> As a kernel developer I have _no_ idea what this is trying to say at
> all, sorry.

Hmm, wow OK, sorry, and I thought I was being too verbose...

OK so first, linker tables allow for the ability to help simplify initialization
sequences so that you no longer have to add the respective static inline in
header files to do nothing, instead you simply get your init routine for your
feature pegged into the linker table or not at link time. If enabling your
feature does not require structural changes, you could then safely enable
compiling this feature all the time, and only allow linking when the feature
was enabled. We don't have an easy way to express this in our build system,
the new targets added lets you accomplish this.

> What is a "specially annotated target"?

The ones listed below, table-obj-y and table-lib-y

> Who is forcing it to be built?

It would be up to maintainers for each subsystem/feature to decide if they want
to use the new targets or not within their subsystem.

> What does it mean if it isn't built?

If you have CONFIG_BUILD_AVOID_BITROT enabled and some code using the special
targets do not get built it means the dependencies it has were not met.

> > +
> > +     include/linux/tables.h
> > +
> > +     The special targets supported are:
> > +
> > +       o table-obj-y
> > +       o table-lib-y
> 
> What does this mean to me as a developer?

It mean you can count on a bit more build test coverage by
CONFIG_BUILD_AVOID_BITROT users. Using table-obj-y is functionally
equivalent to doing:

extra-y += foo.o
obj-y += foo.o

The above new targets are just short hand annotations for the same.  We could
actually use another shorthand prefix other than table-, however linker tables
help making more of these type of targets possible. For instance, on 
initialiation
sequences you no longer have to add each line for each feature onto a set 
routine,
rather you just get the initialization routine linked in or not. This lets us 
avoid
cluttering C code and header code with #idefs, and as a side consequences also
allows more targets to be compiled without implicating functionality.

As a developer you should take care to to use table-obj-y, or table-lib-y only 
if
you are certain the target does not require structural changes.

> What does it mean to a user
> who wants to figure out if it should be enabled or not?

It depends on their build system capability and their goals. If they wish
to be able to report build bugs and have a decent build system they can
enable this. Otherwise they should disable it.

> > +     Say Y if you have a decent build machine and would like to help test
> > +     building code for more subsystems. Say N if you do you not have a
> > +     good build machine or only want to compile what you've enabled for
> > +     your kernel.
> 
> How does this test different subsystems?

By enabling this feature you compile kconfig symbols that typically are
disabled by most users and which have been identified by maintainers as
needing more build testing love. The extra kconfig symbols built are
only built if the dependencies for them are met.

Maintainers for subsystems would have to identify if they have key pieces of
software that typically get disabled, and that enabling them would not incur or
require structural changes, which can use more build test love.

> How does disabling it not test them?

By disabling this feature you only compile kconfig symbols you have
enabled for your kernel.

> Why would I care either way?

You would care if you aware of certain kconfig symbols that do not
get much build test love.

> > +
> > +     Enabling this option never increases the size of your kernel.
> 
> Then what does it do?  Just burn electricity for no reason?

It enables maintainers to annotate through the build system certain
kconfig symbol which should be built if CONFIG_BUILD_AVOID_BITROT is
enabled and its symbol dependencies are met.

  Luis

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