[PATCH 2/2] ppc64le live patch: get rid of mini stack frame
From: Torsten Duwe
Date: Wed Mar 09 2016 - 12:30:29 EST
After the mini stack frame is no longer required for TOC storage, it can
be eliminated iff the functionality of klp_return_helper, which required
a stack frame for the extra return address previously, is carried out
by the replacement function now. This requires _every_ live patch replacement
function to execute the following (or similar) sequence of machine instructions
just before every return to the original caller:
ld r0, 0(r1) /* use back link to find caller's frame */
lwa r2, 12(r0) /* Load from CR+4, offset of TOC w.r.t LR */
ld r0, LRSAVE(r0) /* get the real return address */
add r2, r2, r0 /* Add the current LR to offset */
Signed-off-by: Torsten Duwe <duwe@xxxxxxx>
This is solution 1 now.
Do we really want that? I don't think so; this is merely to illustrate
what the alternative to klp_return_helper and its extra stack frame would
look like. Hence, I didn't test yet whether all the details are correct.
@@ -1277,21 +1277,11 @@ ftrace_call:
* For a local call, restore this TOC after calling the patch function.
* For a global call, it does not matter what we restore here,
* since the global caller does its own restore right afterwards,
- * anyway. Just insert a klp_return_helper frame in any case,
- * so a patch function can always count on the changed stack offsets.
- * The patch introduces a frame such that from the patched function
- * we return back to klp_return helper. For ABI compliance r12,
- * lr and LRSAVE(r1) contain the address of klp_return_helper.
- * We loaded ctr with the address of the patched function earlier
+ * anyway. Just prepare here the TOC restore in patch functions.
+ * We loaded ctr with the address of the patched function earlier.
subf r0, r0, r2 /* Calculate offset from current TOC to LR */
stw r0, 12(r1) /* and save it in CR+4 */
- stdu r1, -32(r1) /* open new mini stack frame */
- bl 5f
-5: mflr r12
- addi r12, r12, (klp_return_helper + 4 - .)@l
- std r12, LRSAVE(r1)
- mtlr r12
mfctr r12 /* allow for TOC calculation in newfunc */
@@ -1313,25 +1303,6 @@ _GLOBAL(ftrace_graph_stub)
-/* Helper function for local calls that are becoming global
- * due to live patching.
- * We can't simply patch the NOP after the original call,
- * because, depending on the consistency model, some kernel
- * threads may still have called the original, local function
- * *without* saving their TOC in the respective stack frame slot,
- * so the decision is made per-thread during function return by
- * maybe inserting a klp_return_helper frame or not.
- addi r1, r1, 32 /* destroy mini stack frame */
- lwa r2, 12(r1) /* Load from CR+4, offset of TOC w.r.t LR */
- ld r0, LRSAVE(r1) /* get the real return address */
- add r2, r2, r0 /* Add the current LR to offset */
- mtlr r0