[RFC][PATCH 3/9] jump_label, x86: Remove init NOP optimization

From: Peter Zijlstra
Date: Mon Oct 07 2019 - 07:23:59 EST


Instead of checking if the emitted (default) NOP is the ideal NOP, and
conditionally rewrite the NOP, just rewrite the NOP.

This shouldn't be a problem because init / module_load uses
text_poke_early() which is cheap and this saves us from having to go
figure out which NOP to compare against when we go variable size.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
arch/x86/kernel/jump_label.c | 19 ++++---------------
1 file changed, 4 insertions(+), 15 deletions(-)

--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -137,21 +137,10 @@ __init_or_module void arch_jump_label_tr
enum jump_label_type type)
{
/*
- * This function is called at boot up and when modules are
- * first loaded. Check if the default nop, the one that is
- * inserted at compile time, is the ideal nop. If it is, then
- * we do not need to update the nop, and we can leave it as is.
- * If it is not, then we need to update the nop to the ideal nop.
+ * Rewrite the NOP on init / module-load to ensure we got the ideal
+ * nop. Don't bother with trying to figure out what size and what nop
+ * it should be for now, simply do an unconditional rewrite.
*/
- if (jlstate == JL_STATE_START) {
- const unsigned char default_nop[] = { STATIC_KEY_INIT_NOP };
- const unsigned char *ideal_nop = ideal_nops[NOP_ATOMIC5];
-
- if (memcmp(ideal_nop, default_nop, 5) != 0)
- jlstate = JL_STATE_UPDATE;
- else
- jlstate = JL_STATE_NO_UPDATE;
- }
- if (jlstate == JL_STATE_UPDATE)
+ if (jlstate == JL_STATE_UPDATE || jlstate == JL_STATE_START)
jump_label_transform(entry, type, 1);
}