Hi Nick,
Nicholas Piggin wrote:
This warns and prevents tracing attempted in a real-mode context.
Is this something you're seeing often? Last time we looked at this, KVM
was the biggest offender and we introduced paca->ftrace_enabled as a way
to disable ftrace while in KVM code.
While this is cheap when handling ftrace_regs_caller() as done in this
patch, for simple function tracing (see below), we will have to grab the
MSR which will slow things down slightly.
Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
arch/powerpc/kernel/trace/ftrace.c | 3 +++
.../powerpc/kernel/trace/ftrace_64_mprofile.S | 19 +++++++++++++++----
2 files changed, 18 insertions(+), 4 deletions(-)
diff --git a/arch/powerpc/kernel/trace/ftrace.c
b/arch/powerpc/kernel/trace/ftrace.c
index 7ea0ca044b65..ef965815fcb9 100644
--- a/arch/powerpc/kernel/trace/ftrace.c
+++ b/arch/powerpc/kernel/trace/ftrace.c
@@ -949,6 +949,9 @@ unsigned long prepare_ftrace_return(unsigned long parent,
unsigned long ip,
{
unsigned long return_hooker;
+ if (WARN_ON_ONCE((mfmsr() & (MSR_IR|MSR_DR)) != (MSR_IR|MSR_DR)))
+ goto out;
+
This is called on function entry to redirect function return to a
trampoline if needed. I am not sure if we have (or will have) too many C
functions that disable MSR_IR|MSR_DR. Unless the number of such
functions is large, it might be preferable to mark specific functions as
notrace.
if (unlikely(ftrace_graph_is_dead()))
goto out;
diff --git a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
index f9fd5f743eba..6205f15cb603 100644
--- a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
+++ b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
@@ -51,16 +51,21 @@ _GLOBAL(ftrace_regs_caller)
SAVE_10GPRS(12, r1)
SAVE_10GPRS(22, r1)
- /* Save previous stack pointer (r1) */
- addi r8, r1, SWITCH_FRAME_SIZE
- std r8, GPR1(r1)
-
/* Load special regs for save below */
mfmsr r8
mfctr r9
mfxer r10
mfcr r11
+ /* Shouldn't be called in real mode */
+ andi. r3,r8,(MSR_IR|MSR_DR)
+ cmpdi r3,(MSR_IR|MSR_DR)
+ bne ftrace_bad_realmode
+
+ /* Save previous stack pointer (r1) */
+ addi r8, r1, SWITCH_FRAME_SIZE
+ std r8, GPR1(r1)
+
This stomps on the MSR value in r8, which is saved into pt_regs further
below.
You'll also have to handle ftrace_caller() which is used for simple
function tracing. We don't read the MSR there today, but that will be
needed if we want to suppress tracing.
- Naveen
