powerpc/modules: Load modules closer to kernel text
On book3s/32, when STRICT_KERNEL_RWX is selected, modules are
allocated on the segment just before kernel text, ie on the
0xb0000000-0xbfffffff when PAGE_OFFSET is 0xc0000000.
On the 8xx, TASK_SIZE is 0x80000000. The space between TASK_SIZE and
PAGE_OFFSET is not used and could be used for modules.
The idea comes from ARM architecture.
Having modules just below PAGE_OFFSET offers an opportunity to
minimise the distance between kernel text and modules and avoid
trampolines in modules to access kernel functions or other module
functions.
When MODULES_VADDR is defined, powerpc has it's own module_alloc()
function. In that function, first try to allocate the module
above the limit defined by '_etext - 32M'. Then if the allocation
fails, fallback to the entire MODULES area.
DEBUG logs in module_32.c without the patch:
[ 1572.588822] module_32: Applying ADD relocate section 13 to 12
[ 1572.588891] module_32: Doing plt for call to 0xc00671a4 at 0xcae04024
[ 1572.588964] module_32: Initialized plt for 0xc00671a4 at
cae04000
[ 1572.589037] module_32: REL24 value =
CAE04000. location =
CAE04024
[ 1572.589110] module_32: Location before:
48000001.
[ 1572.589171] module_32: Location after:
4BFFFFDD.
[ 1572.589231] module_32: ie. jump to
03FFFFDC+
CAE04024 =
CEE04000
[ 1572.589317] module_32: Applying ADD relocate section 15 to 14
[ 1572.589386] module_32: Doing plt for call to 0xc00671a4 at 0xcadfc018
[ 1572.589457] module_32: Initialized plt for 0xc00671a4 at
cadfc000
[ 1572.589529] module_32: REL24 value =
CADFC000. location =
CADFC018
[ 1572.589601] module_32: Location before:
48000000.
[ 1572.589661] module_32: Location after:
4BFFFFE8.
[ 1572.589723] module_32: ie. jump to
03FFFFE8+
CADFC018 =
CEDFC000
With the patch:
[ 279.404671] module_32: Applying ADD relocate section 13 to 12
[ 279.404741] module_32: REL24 value =
C00671B4. location =
BF808024
[ 279.404814] module_32: Location before:
48000001.
[ 279.404874] module_32: Location after:
4885F191.
[ 279.404933] module_32: ie. jump to
0085F190+
BF808024 =
C00671B4
[ 279.405016] module_32: Applying ADD relocate section 15 to 14
[ 279.405085] module_32: REL24 value =
C00671B4. location =
BF800018
[ 279.405156] module_32: Location before:
48000000.
[ 279.405215] module_32: Location after:
4886719C.
[ 279.405275] module_32: ie. jump to
0086719C+
BF800018 =
C00671B4
We see that with the patch, no plt entries are set.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0c3d5cb8a4dfdf6ca1b8aeb385c01470d6628d55.1617283827.git.christophe.leroy@csgroup.eu