powerpc64/ftrace: Add a field in paca to disable ftrace in unsafe code paths

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_enabled',
which is checked on ftrace entry before continuing. This field can then
be set to zero to disable/pause ftrace, and set to a non-zero value to
resume ftrace.
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/paca.h

@@ -223,6 +223,7 @@ struct paca_struct {
 	u8 hmi_event_available;		/* HMI event is available */
 	u8 hmi_p9_special_emu;		/* HMI P9 special emulation */
 #endif
+	u8 ftrace_enabled;		/* Hard disable ftrace */
 
 	/* Stuff for accurate time accounting */
 	struct cpu_accounting_data accounting;

arch/powerpc/kernel/asm-offsets.c

@@ -180,6 +180,7 @@ int main(void)
 	OFFSET(PACAKMSR, paca_struct, kernel_msr);
 	OFFSET(PACAIRQSOFTMASK, paca_struct, irq_soft_mask);
 	OFFSET(PACAIRQHAPPENED, paca_struct, irq_happened);
+	OFFSET(PACA_FTRACE_ENABLED, paca_struct, ftrace_enabled);
 #ifdef CONFIG_PPC_BOOK3S
 	OFFSET(PACACONTEXTID, paca_struct, mm_ctx_id);
 #ifdef CONFIG_PPC_MM_SLICES

arch/powerpc/kernel/setup_64.c

@@ -252,6 +252,9 @@ static void cpu_ready_for_interrupts(void)
 
 	/* Set IR and DR in PACA MSR */
 	get_paca()->kernel_msr = MSR_KERNEL;
+
+	/* We are now ok to enable ftrace */
+	get_paca()->ftrace_enabled = 1;
 }
 
 unsigned long spr_default_dscr = 0;

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_ENABLED(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

arch/powerpc/kernel/trace/ftrace_64_pg.S

@@ -16,6 +16,10 @@
 
 #ifdef CONFIG_DYNAMIC_FTRACE
 _GLOBAL_TOC(ftrace_caller)
+	lbz	r3, PACA_FTRACE_ENABLED(r13)
+	cmpdi	r3, 0
+	beqlr
+
 	/* Taken from output of objdump from lib64/glibc */
 	mflr	r3
 	ld	r11, 0(r1)