[PATCH v6 0/8] implement DYNAMIC_DEBUG_RELATIVE_POINTERS

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

Similar to CONFIG_GENERIC_BUG_RELATIVE_POINTERS that replaces (8 byte)
const char* members by (4 byte) signed offsets from the bug_entry,
this implements the similar thing for struct _ddebug, the descriptors
underlying pr_debug() and friends in a CONFIG_DYNAMIC_DEBUG kernel.

Since struct _ddebug has four string members, we save 16 byte per
instance. The total savings seem to be comparable to what is saved by
an architecture selecting GENERIC_BUG_RELATIVE_POINTERS (see patch 8
for some numbers for a common distro config).

While refreshing these patches, which were orignally just targeted at
x86-64, it occured to me that despite the implementation relying on
inline asm, there's nothing x86 specific about it, and indeed it seems
to work out-of-the-box for ppc64 and arm64 as well, but those have
only been compile-tested.

The first 6 patches are rather pedestrian preparations. The fun stuff
is in patch 7, and the last patch is just the minimal boilerplate to
hook up the asm-generic header and selecting
HAVE_DYNAMIC_DEBUG_RELATIVE_POINTERS on x86-64. Doing arm64 and ppc64
would be similar 2-line patches.

Unfortunately, the "fun stuff" includes some magic that breaks the
build in some corner cases with older gcc (and some not-so-magic that
clang only supports since version 9 for non-x86 targets). So in this
v6, the config option DYNAMIC_DEBUG_RELATIVE_POINTERS has been made
opt-in, depending on the arch-selected
HAVE_DYNAMIC_DEBUG_RELATIVE_POINTERS, instead of the arch selecting it
directly. Also, I've set a lower bound for the gcc and clang versions,
so that hopefully nobody should be able to select the option and have
the build break.


Rasmus Villemoes (8):
  linux/device.h: use unique identifier for each struct _ddebug
  linux/net.h: use unique identifier for each struct _ddebug
  linux/printk.h: use unique identifier for each struct _ddebug
  dynamic_debug: introduce accessors for string members of struct
    _ddebug
  dynamic_debug: drop use of bitfields in struct _ddebug
  dynamic_debug: introduce CONFIG_DYNAMIC_DEBUG_RELATIVE_POINTERS
  dynamic_debug: add asm-generic implementation for
    DYNAMIC_DEBUG_RELATIVE_POINTERS
  x86-64: select HAVE_DYNAMIC_DEBUG_RELATIVE_POINTERS

 arch/x86/Kconfig                            |   1 +
 arch/x86/entry/vdso/vdso32/vclock_gettime.c |   1 +
 arch/x86/include/asm/Kbuild                 |   1 +
 include/asm-generic/dynamic_debug.h         | 116 ++++++++++++++++++++
 include/linux/device.h                      |   4 +-
 include/linux/dynamic_debug.h               |  26 +++--
 include/linux/jump_label.h                  |   2 +
 include/linux/net.h                         |   4 +-
 include/linux/printk.h                      |   4 +-
 lib/Kconfig.debug                           |  16 +++
 lib/dynamic_debug.c                         | 111 ++++++++++++++-----
 11 files changed, 246 insertions(+), 40 deletions(-)
 create mode 100644 include/asm-generic/dynamic_debug.h

-- 
2.20.1