- Commit missing part of "bt" fix: store PC register in pcb_context struct

in cpu_switch and use it in stack_trace function later. pcb_regs contains
    state of the process stored by exception handler and therefor is not
    valid for sleeping processes.

sys/mips/include/pcb.h

@@ -50,7 +50,7 @@
 struct pcb
 {
 	struct trapframe pcb_regs;	/* saved CPU and registers */
-	__register_t pcb_context[13];	/* kernel context for resume */
+	__register_t pcb_context[14];	/* kernel context for resume */
 	int	pcb_onfault;		/* for copyin/copyout faults */
 };
 
@@ -70,6 +70,7 @@ struct pcb
 #define PCB_REG_RA   10
 #define PCB_REG_SR   11
 #define PCB_REG_GP   12
+#define PCB_REG_PC   13
 
 
 #ifdef _KERNEL

sys/mips/include/regnum.h

@@ -63,8 +63,7 @@
 #define PREG_RA   10
 #define PREG_SR   11
 #define PREG_GP   12
-
-
+#define PREG_PC   13
 
 /*
  * Location of the saved registers relative to ZERO.

sys/mips/mips/db_trace.c

@@ -420,7 +420,7 @@ db_trace_thread(struct thread *thr, int count)
 	else {
 		ctx = thr->td_pcb;
 		sp = (register_t)ctx->pcb_context[PREG_SP];
-		pc = (register_t)ctx->pcb_regs.pc;
+		pc = (register_t)ctx->pcb_context[PREG_PC];
 		ra = (register_t)ctx->pcb_context[PREG_RA];
 	}
 

sys/mips/mips/swtch.S

@@ -313,6 +313,10 @@ NON_LEAF(cpu_switch, STAND_FRAME_SIZE, ra)
 	SAVE_U_PCB_CONTEXT(ra, PREG_RA, a0)		# save return address
 	SAVE_U_PCB_CONTEXT(t0, PREG_SR, a0)		# save status register
 	SAVE_U_PCB_CONTEXT(gp, PREG_GP, a0)
+	jal	getpc
+	nop
+getpc:
+	SAVE_U_PCB_CONTEXT(ra, PREG_PC, a0)		# save return address
 	/*
 	 * FREEBSD_DEVELOPERS_FIXME:
 	 * In case there are CPU-specific registers that need