arm64: alternatives: add shared NOP callback

For each instance of an alternative, the compiler outputs a distinct
copy of the alternative instructions into a subsection. As the compiler
doesn't have special knowledge of alternatives, it cannot coalesce these
to save space.

In a defconfig kernel built with GCC 12.1.0, there are approximately
10,000 instances of alternative_has_feature_likely(), where the
replacement instruction is always a NOP. As NOPs are
position-independent, we don't need a unique copy per alternative
sequence.

This patch adds a callback to patch an alternative sequence with NOPs,
and make use of this in alternative_has_feature_likely(). So that this
can be used for other sites in future, this is written to patch multiple
instructions up to the original sequence length.

For NVHE, an alias is added to image-vars.h.

For modules, the callback is exported. Note that as modules are loaded
within 2GiB of the kernel, an alt_instr entry in a module can always
refer directly to the callback, and no special handling is necessary.

When building with GCC 12.1.0, the vmlinux is ~158KiB smaller, though
the resulting Image size is unchanged due to alignment constraints and
padding:

| % ls -al vmlinux-*
| -rwxr-xr-x 1 mark mark 134644592 Sep  1 14:52 vmlinux-after
| -rwxr-xr-x 1 mark mark 134486232 Sep  1 14:50 vmlinux-before
| % ls -al Image-*
| -rw-r--r-- 1 mark mark 37108224 Sep  1 14:52 Image-after
| -rw-r--r-- 1 mark mark 37108224 Sep  1 14:50 Image-before

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220912162210.3626215-9-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index 9a071a5fcb67..5a904d4e98ea 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -263,3 +263,11 @@ void apply_alternatives_module(void *start, size_t length)
 	__apply_alternatives(&region, true, &all_capabilities[0]);
 }
 #endif
+
+noinstr void alt_cb_patch_nops(struct alt_instr *alt, __le32 *origptr,
+			       __le32 *updptr, int nr_inst)
+{
+	for (int i = 0; i < nr_inst; i++)
+		updptr[i] = cpu_to_le32(aarch64_insn_gen_nop());
+}
+EXPORT_SYMBOL(alt_cb_patch_nops);