[PATCH v2 1/5] powerpc64/ftrace: Add a field in paca to disable ftrace in unsafe code paths

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

We have some C code that we call into from real mode where we cannot
take any exceptions. Though the C functions themselves are mostly safe,
if these functions are traced, there is a possibility that we may take
an exception. For instance, in certain conditions, the ftrace code uses
WARN(), which uses a 'trap' to do its job.

For such scenarios, introduce a new field in paca 'ftrace_disabled',
which is checked on ftrace entry before continuing. This field can then
be set to a non-zero value to disable/pause ftrace, and reset to zero to
resume ftrace.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
Changes since v1:
- Do not guard the paca field check within CONFIG_KVM.
- Move the early return code out of the usual path to optimize for 
  better cache performance in the normal case when ftrace is enabled.

 arch/powerpc/include/asm/paca.h                |  1 +
 arch/powerpc/kernel/asm-offsets.c              |  1 +
 arch/powerpc/kernel/trace/ftrace_64_mprofile.S | 14 ++++++++++++++
 arch/powerpc/kernel/trace/ftrace_64_pg.S       |  4 ++++
 4 files changed, 20 insertions(+)

diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
index d2bf71dddbef..4f47adc2a408 100644
--- a/arch/powerpc/include/asm/paca.h
+++ b/arch/powerpc/include/asm/paca.h
@@ -211,6 +211,7 @@ struct paca_struct {
 	u16 in_mce;
 	u8 hmi_event_available;		/* HMI event is available */
 	u8 hmi_p9_special_emu;		/* HMI P9 special emulation */
+	u8 ftrace_disabled;
 #endif
 
 	/* Stuff for accurate time accounting */
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index ea5eb91b836e..8e4fc96ff6bc 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -240,6 +240,7 @@ int main(void)
 	OFFSET(PACA_RFI_FLUSH_FALLBACK_AREA, paca_struct, rfi_flush_fallback_area);
 	OFFSET(PACA_EXRFI, paca_struct, exrfi);
 	OFFSET(PACA_L1D_FLUSH_SIZE, paca_struct, l1d_flush_size);
+	OFFSET(PACA_FTRACE_DISABLED, paca_struct, ftrace_disabled);
 
 #endif
 	OFFSET(PACAHWCPUID, paca_struct, hw_cpu_id);
diff --git a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
index 3f3e81852422..8f2380304ef1 100644
--- a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
+++ b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
@@ -47,6 +47,12 @@ _GLOBAL(ftrace_caller)
 	/* Save all gprs to pt_regs */
 	SAVE_GPR(0, r1)
 	SAVE_10GPRS(2, r1)
+
+	/* Ok to continue? */
+	lbz	r3, PACA_FTRACE_DISABLED(r13)
+	cmpdi	r3, 0
+	beq	ftrace_no_trace
+
 	SAVE_10GPRS(12, r1)
 	SAVE_10GPRS(22, r1)
 
@@ -168,6 +174,14 @@ _GLOBAL(ftrace_graph_stub)
 _GLOBAL(ftrace_stub)
 	blr
 
+ftrace_no_trace:
+	mflr	r3
+	mtctr	r3
+	REST_GPR(3, r1)
+	addi	r1, r1, SWITCH_FRAME_SIZE
+	mtlr	r0
+	bctr
+
 #ifdef CONFIG_LIVEPATCH
 	/*
 	 * This function runs in the mcount context, between two functions. As
diff --git a/arch/powerpc/kernel/trace/ftrace_64_pg.S b/arch/powerpc/kernel/trace/ftrace_64_pg.S
index f095358da96e..3d8186832a34 100644
--- a/arch/powerpc/kernel/trace/ftrace_64_pg.S
+++ b/arch/powerpc/kernel/trace/ftrace_64_pg.S
@@ -16,6 +16,10 @@
 
 #ifdef CONFIG_DYNAMIC_FTRACE
 _GLOBAL_TOC(ftrace_caller)
+	lbz	r3, PACA_FTRACE_DISABLED(r13)
+	cmpdi	r3, 0
+	bnelr
+
 	/* Taken from output of objdump from lib64/glibc */
 	mflr	r3
 	ld	r11, 0(r1)
-- 
2.16.2