x86/speculation/mds: Add mitigation control for MDS

[mirror_ubuntu-bionic-kernel.git] / Documentation / dev-tools / sparse.rst

.. Copyright 2004 Linus Torvalds
.. Copyright 2004 Pavel Machek <pavel@ucw.cz>
.. Copyright 2006 Bob Copeland <me@bobcopeland.com>

Sparse
======

Sparse is a semantic checker for C programs; it can be used to find a
number of potential problems with kernel code.  See
https://lwn.net/Articles/689907/ for an overview of sparse; this document
contains some kernel-specific sparse information.


Using sparse for typechecking
-----------------------------

"__bitwise" is a type attribute, so you have to do something like this::

        typedef int __bitwise pm_request_t;

        enum pm_request {
                PM_SUSPEND = (__force pm_request_t) 1,
                PM_RESUME = (__force pm_request_t) 2
        };

which makes PM_SUSPEND and PM_RESUME "bitwise" integers (the "__force" is
there because sparse will complain about casting to/from a bitwise type,
but in this case we really _do_ want to force the conversion). And because
the enum values are all the same type, now "enum pm_request" will be that
type too.

And with gcc, all the "__bitwise"/"__force stuff" goes away, and it all
ends up looking just like integers to gcc.

Quite frankly, you don't need the enum there. The above all really just
boils down to one special "int __bitwise" type.

So the simpler way is to just do::

        typedef int __bitwise pm_request_t;

        #define PM_SUSPEND ((__force pm_request_t) 1)
        #define PM_RESUME ((__force pm_request_t) 2)

and you now have all the infrastructure needed for strict typechecking.

One small note: the constant integer "0" is special. You can use a
constant zero as a bitwise integer type without sparse ever complaining.
This is because "bitwise" (as the name implies) was designed for making
sure that bitwise types don't get mixed up (little-endian vs big-endian
vs cpu-endian vs whatever), and there the constant "0" really _is_
special.

Using sparse for lock checking
------------------------------

The following macros are undefined for gcc and defined during a sparse
run to use the "context" tracking feature of sparse, applied to
locking.  These annotations tell sparse when a lock is held, with
regard to the annotated function's entry and exit.

__must_hold - The specified lock is held on function entry and exit.

__acquires - The specified lock is held on function exit, but not entry.

__releases - The specified lock is held on function entry, but not exit.

If the function enters and exits without the lock held, acquiring and
releasing the lock inside the function in a balanced way, no
annotation is needed.  The tree annotations above are for cases where
sparse would otherwise report a context imbalance.

Getting sparse
--------------

You can get latest released versions from the Sparse homepage at
https://sparse.wiki.kernel.org/index.php/Main_Page

Alternatively, you can get snapshots of the latest development version
of sparse using git to clone::

        git://git.kernel.org/pub/scm/devel/sparse/sparse.git

DaveJ has hourly generated tarballs of the git tree available at::

        http://www.codemonkey.org.uk/projects/git-snapshots/sparse/


Once you have it, just do::

        make
        make install

as a regular user, and it will install sparse in your ~/bin directory.

Using sparse
------------

Do a kernel make with "make C=1" to run sparse on all the C files that get
recompiled, or use "make C=2" to run sparse on the files whether they need to
be recompiled or not.  The latter is a fast way to check the whole tree if you
have already built it.

The optional make variable CF can be used to pass arguments to sparse.  The
build system passes -Wbitwise to sparse automatically.