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Subject[PATCH v3 5/5] powerpc: kprobes: prefer ftrace when probing function entry
Date
KPROBES_ON_FTRACE avoids much of the overhead with regular kprobes as it
eliminates the need for a trap, as well as the need to emulate or
single-step instructions.

Though OPTPROBES provides us with similar performance, we have limited
optprobes trampoline slots. As such, when asked to probe at a function
entry, default to using the ftrace infrastructure.

With:
# cd /sys/kernel/debug/tracing
# echo 'p _do_fork' > kprobe_events

before patch:
# cat ../kprobes/list
c0000000000daf08 k _do_fork+0x8 [DISABLED]
c000000000044fc0 k kretprobe_trampoline+0x0 [OPTIMIZED]

and after patch:
# cat ../kprobes/list
c0000000000d074c k _do_fork+0xc [DISABLED][FTRACE]
c0000000000412b0 k kretprobe_trampoline+0x0 [OPTIMIZED]

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
arch/powerpc/kernel/kprobes.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index b78b274e1d6e..23d19678a56f 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -49,8 +49,21 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
#ifdef PPC64_ELF_ABI_v2
/* PPC64 ABIv2 needs local entry point */
addr = (kprobe_opcode_t *)kallsyms_lookup_name(name);
- if (addr && !offset)
- addr = (kprobe_opcode_t *)ppc_function_entry(addr);
+ if (addr && !offset) {
+#ifdef CONFIG_KPROBES_ON_FTRACE
+ unsigned long faddr;
+ /*
+ * Per livepatch.h, ftrace location is always within the first
+ * 16 bytes of a function on powerpc with -mprofile-kernel.
+ */
+ faddr = ftrace_location_range((unsigned long)addr,
+ (unsigned long)addr + 16);
+ if (faddr)
+ addr = (kprobe_opcode_t *)faddr;
+ else
+#endif
+ addr = (kprobe_opcode_t *)ppc_function_entry(addr);
+ }
#elif defined(PPC64_ELF_ABI_v1)
/*
* 64bit powerpc ABIv1 uses function descriptors:
--
2.12.1
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