powerpc: Save Come-From Address Register (CFAR) in exception frame

Recent 64-bit server processors (POWER6 and POWER7) have a "Come-From
Address Register" (CFAR), that records the address of the most recent
branch or rfid (return from interrupt) instruction for debugging purposes.

This saves the value of the CFAR in the exception entry code and stores
it in the exception frame.  We also make xmon print the CFAR value in
its register dump code.

Rather than extend the pt_regs struct at this time, we steal the orig_gpr3
field, which is only used for system calls, and use it for the CFAR value
for all exceptions/interrupts other than system calls.  This means we
don't save the CFAR on system calls, which is not a great problem since
system calls tend not to happen unexpectedly, and also avoids adding the
overhead of reading the CFAR to the system call entry path.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/cputable.h

@@ -180,6 +180,7 @@ extern const char *powerpc_base_platform;
 
 
 #define CPU_FTR_HVMODE_206		LONG_ASM_CONST(0x0000000800000000)
+#define CPU_FTR_CFAR			LONG_ASM_CONST(0x0000001000000000)
 #define CPU_FTR_IABR			LONG_ASM_CONST(0x0000002000000000)
 #define CPU_FTR_MMCRA			LONG_ASM_CONST(0x0000004000000000)
 #define CPU_FTR_CTRL			LONG_ASM_CONST(0x0000008000000000)
@@ -412,7 +413,7 @@ extern const char *powerpc_base_platform;
 	    CPU_FTR_COHERENT_ICACHE | \
 	    CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
 	    CPU_FTR_DSCR | CPU_FTR_UNALIGNED_LD_STD | \
-	    CPU_FTR_STCX_CHECKS_ADDRESS | CPU_FTR_POPCNTB)
+	    CPU_FTR_STCX_CHECKS_ADDRESS | CPU_FTR_POPCNTB | CPU_FTR_CFAR)
 #define CPU_FTRS_POWER7 (CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
 	    CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | CPU_FTR_HVMODE_206 |\
 	    CPU_FTR_MMCRA | CPU_FTR_SMT | \
@@ -420,7 +421,7 @@ extern const char *powerpc_base_platform;
 	    CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
 	    CPU_FTR_DSCR | CPU_FTR_SAO  | CPU_FTR_ASYM_SMT | \
 	    CPU_FTR_STCX_CHECKS_ADDRESS | CPU_FTR_POPCNTB | CPU_FTR_POPCNTD | \
-	    CPU_FTR_ICSWX)
+	    CPU_FTR_ICSWX | CPU_FTR_CFAR)
 #define CPU_FTRS_CELL	(CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
 	    CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
 	    CPU_FTR_ALTIVEC_COMP | CPU_FTR_MMCRA | CPU_FTR_SMT | \

arch/powerpc/include/asm/exception-64s.h

@@ -46,6 +46,7 @@
 #define EX_CCR		60
 #define EX_R3		64
 #define EX_LR		72
+#define EX_CFAR		80
 
 /*
  * We're short on space and time in the exception prolog, so we can't
@@ -66,6 +67,10 @@
 	std	r10,area+EX_R10(r13);					\
 	std	r11,area+EX_R11(r13);					\
 	std	r12,area+EX_R12(r13);					\
+	BEGIN_FTR_SECTION_NESTED(66);					\
+	mfspr	r10,SPRN_CFAR;						\
+	std	r10,area+EX_CFAR(r13);					\
+	END_FTR_SECTION_NESTED(CPU_FTR_CFAR, CPU_FTR_CFAR, 66);		\
 	GET_SCRATCH0(r9);						\
 	std	r9,area+EX_R13(r13);					\
 	mfcr	r9
@@ -130,6 +135,10 @@
 	std	r9,GPR11(r1);						   \
 	std	r10,GPR12(r1);						   \
 	std	r11,GPR13(r1);						   \
+	BEGIN_FTR_SECTION_NESTED(66);					   \
+	ld	r10,area+EX_CFAR(r13);					   \
+	std	r10,ORIG_GPR3(r1);					   \
+	END_FTR_SECTION_NESTED(CPU_FTR_CFAR, CPU_FTR_CFAR, 66);		   \
 	ld	r2,PACATOC(r13);	/* get kernel TOC into r2	*/ \
 	mflr	r9;			/* save LR in stackframe	*/ \
 	std	r9,_LINK(r1);						   \

arch/powerpc/include/asm/paca.h

@@ -92,9 +92,9 @@ struct paca_struct {
 	 * Now, starting in cacheline 2, the exception save areas
 	 */
 	/* used for most interrupts/exceptions */
-	u64 exgen[10] __attribute__((aligned(0x80)));
-	u64 exmc[10];		/* used for machine checks */
-	u64 exslb[10];		/* used for SLB/segment table misses
+	u64 exgen[11] __attribute__((aligned(0x80)));
+	u64 exmc[11];		/* used for machine checks */
+	u64 exslb[11];		/* used for SLB/segment table misses
  				 * on the linear mapping */
 	/* SLB related definitions */
 	u16 vmalloc_sllp;

arch/powerpc/include/asm/reg.h

@@ -188,6 +188,7 @@
 
 #define SPRN_CTR	0x009	/* Count Register */
 #define SPRN_DSCR	0x11
+#define SPRN_CFAR	0x1c	/* Come From Address Register */
 #define SPRN_ACOP	0x1F	/* Available Coprocessor Register */
 #define SPRN_CTRLF	0x088
 #define SPRN_CTRLT	0x098

arch/powerpc/kernel/exceptions-64s.S

@@ -474,6 +474,10 @@ bad_stack:
 	std	r9,GPR11(r1)
 	std	r10,GPR12(r1)
 	std	r11,GPR13(r1)
+BEGIN_FTR_SECTION
+	ld	r10,EX_CFAR(r3)
+	std	r10,ORIG_GPR3(r1)
+END_FTR_SECTION_IFSET(CPU_FTR_CFAR)
 	SAVE_8GPRS(14,r1)
 	SAVE_10GPRS(22,r1)
 	lhz	r12,PACA_TRAP_SAVE(r13)

arch/powerpc/xmon/xmon.c

@@ -1497,6 +1497,10 @@ static void prregs(struct pt_regs *fp)
 #endif
 	printf("pc  = ");
 	xmon_print_symbol(fp->nip, " ", "\n");
+	if (TRAP(fp) != 0xc00 && cpu_has_feature(CPU_FTR_CFAR)) {
+		printf("cfar= ");
+		xmon_print_symbol(fp->orig_gpr3, " ", "\n");
+	}
 	printf("lr  = ");
 	xmon_print_symbol(fp->link, " ", "\n");
 	printf("msr = "REG"   cr  = %.8lx\n", fp->msr, fp->ccr);