arm64: remove pcb_pc

The program counter field in the PCB is written in exactly one place,
makectx(), upon entry to the debugger. For threads other than curthread,
its value will be empty, or bogus. Rather than writing to this field in
more places, it can be removed in favor of using the value in the link
register.

To make this clearer, pcb->pcb_x[30] is renamed to pcb->pcb_lr, similar
to what already exists in struct trapframe. Also, prefer lr to x30 in
assembly, as it better conveys intention.

This improves PC_REGS() for kdb_thread != curthread. It is required for
a functional gdb(4) stub, fixing the output of `info threads`, in
particular.

The space occupied by pcb_pc is retained, for compatibility with kgdb.

Reviewed by:	markj, jhb
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D27720

9 files changed, 18 insertions, 17 deletions

diff --git a/sys/arm64/arm64/db_trace.c b/sys/arm64/arm64/db_trace.c
index 42f63ffdf4be..1743d7765861 100644
--- a/sys/arm64/arm64/db_trace.c
+++ b/sys/arm64/arm64/db_trace.c
@@ -109,7 +109,7 @@ db_trace_thread(struct thread *thr, int count)
 
 		frame.sp = (uintptr_t)ctx->pcb_sp;
 		frame.fp = (uintptr_t)ctx->pcb_x[29];
-		frame.pc = (uintptr_t)ctx->pcb_x[30];
+		frame.pc = (uintptr_t)ctx->pcb_lr;
 		db_stack_trace_cmd(thr, &frame);
 	} else
 		db_trace_self();
diff --git a/sys/arm64/arm64/exception.S b/sys/arm64/arm64/exception.S
index 123f73b49734..bcb444ef2f55 100644
--- a/sys/arm64/arm64/exception.S
+++ b/sys/arm64/arm64/exception.S
@@ -39,7 +39,7 @@ __FBSDID("$FreeBSD$");
 	sub	sp, sp, #128
 .endif
 	sub	sp, sp, #(TF_SIZE + 16)
-	stp	x29, x30, [sp, #(TF_SIZE)]
+	stp	x29, lr, [sp, #(TF_SIZE)]
 	stp	x28, x29, [sp, #(TF_X + 28 * 8)]
 	stp	x26, x27, [sp, #(TF_X + 26 * 8)]
 	stp	x24, x25, [sp, #(TF_X + 24 * 8)]
diff --git a/sys/arm64/arm64/genassym.c b/sys/arm64/arm64/genassym.c
index c211d121c06e..1575a0158dec 100644
--- a/sys/arm64/arm64/genassym.c
+++ b/sys/arm64/arm64/genassym.c
@@ -60,6 +60,7 @@ ASSYM(PC_SSBD, offsetof(struct pcpu, pc_ssbd));
 ASSYM(PCB_SIZE, roundup2(sizeof(struct pcb), STACKALIGNBYTES + 1));
 ASSYM(PCB_SINGLE_STEP_SHIFT, PCB_SINGLE_STEP_SHIFT);
 ASSYM(PCB_REGS, offsetof(struct pcb, pcb_x));
+ASSYM(PCB_LR, offsetof(struct pcb, pcb_lr));
 ASSYM(PCB_SP, offsetof(struct pcb, pcb_sp));
 ASSYM(PCB_TPIDRRO, offsetof(struct pcb, pcb_tpidrro_el0));
 ASSYM(PCB_ONFAULT, offsetof(struct pcb, pcb_onfault));
diff --git a/sys/arm64/arm64/machdep.c b/sys/arm64/arm64/machdep.c
index 739257bed1db..90fc19d57415 100644
--- a/sys/arm64/arm64/machdep.c
+++ b/sys/arm64/arm64/machdep.c
@@ -732,11 +732,11 @@ makectx(struct trapframe *tf, struct pcb *pcb)
 {
 	int i;
 
-	for (i = 0; i < PCB_LR; i++)
+	for (i = 0; i < nitems(pcb->pcb_x); i++)
 		pcb->pcb_x[i] = tf->tf_x[i];
 
-	pcb->pcb_x[PCB_LR] = tf->tf_lr;
-	pcb->pcb_pc = tf->tf_elr;
+	/* NB: pcb_lr is the PC, see PC_REGS() in db_machdep.h */
+	pcb->pcb_lr = tf->tf_elr;
 	pcb->pcb_sp = tf->tf_sp;
 }
 
diff --git a/sys/arm64/arm64/stack_machdep.c b/sys/arm64/arm64/stack_machdep.c
index 7ca64c9f3b2d..673da833afb4 100644
--- a/sys/arm64/arm64/stack_machdep.c
+++ b/sys/arm64/arm64/stack_machdep.c
@@ -71,7 +71,7 @@ stack_save_td(struct stack *st, struct thread *td)
 
 	frame.sp = td->td_pcb->pcb_sp;
 	frame.fp = td->td_pcb->pcb_x[29];
-	frame.pc = td->td_pcb->pcb_x[30];
+	frame.pc = td->td_pcb->pcb_lr;
 
 	stack_capture(td, st, &frame);
 	return (0);
diff --git a/sys/arm64/arm64/swtch.S b/sys/arm64/arm64/swtch.S
index 144cc0873f68..ee64f89502cc 100644
--- a/sys/arm64/arm64/swtch.S
+++ b/sys/arm64/arm64/swtch.S
@@ -101,7 +101,7 @@ ENTRY(cpu_throw)
 	ldp	x24, x25, [x4, #PCB_REGS + 24 * 8]
 	ldp	x26, x27, [x4, #PCB_REGS + 26 * 8]
 	ldp	x28, x29, [x4, #PCB_REGS + 28 * 8]
-	ldr	x30, [x4, #PCB_REGS + 30 * 8]
+	ldr	lr, [x4, #PCB_LR]
 
 	ret
 END(cpu_throw)
@@ -132,7 +132,7 @@ ENTRY(cpu_switch)
 	stp	x24, x25, [x4, #PCB_REGS + 24 * 8]
 	stp	x26, x27, [x4, #PCB_REGS + 26 * 8]
 	stp	x28, x29, [x4, #PCB_REGS + 28 * 8]
-	str	x30, [x4, #PCB_REGS + 30 * 8]
+	str	lr, [x4, #PCB_LR]
 	/* And the old stack pointer */
 	mov	x5, sp
 	mrs	x6, tpidrro_el0
@@ -198,7 +198,7 @@ ENTRY(cpu_switch)
 	ldp	x24, x25, [x4, #PCB_REGS + 24 * 8]
 	ldp	x26, x27, [x4, #PCB_REGS + 26 * 8]
 	ldp	x28, x29, [x4, #PCB_REGS + 28 * 8]
-	ldr	x30, [x4, #PCB_REGS + 30 * 8]
+	ldr	lr, [x4, #PCB_LR]
 
 	str	xzr, [x4, #PCB_REGS + 18 * 8]
 	ret
@@ -270,7 +270,7 @@ ENTRY(savectx)
 	stp	x24, x25, [x0, #PCB_REGS + 24 * 8]
 	stp	x26, x27, [x0, #PCB_REGS + 26 * 8]
 	stp	x28, x29, [x0, #PCB_REGS + 28 * 8]
-	str	x30, [x0, #PCB_REGS + 30 * 8]
+	str	lr, [x0, #PCB_LR]
 	/* And the old stack pointer */
 	mov	x5, sp
 	mrs	x6, tpidrro_el0
diff --git a/sys/arm64/arm64/vm_machdep.c b/sys/arm64/arm64/vm_machdep.c
index b4a3e596f9ef..6a84dc023562 100644
--- a/sys/arm64/arm64/vm_machdep.c
+++ b/sys/arm64/arm64/vm_machdep.c
@@ -102,7 +102,7 @@ cpu_fork(struct thread *td1, struct proc *p2, struct thread *td2, int flags)
 	/* Set the return value registers for fork() */
 	td2->td_pcb->pcb_x[8] = (uintptr_t)fork_return;
 	td2->td_pcb->pcb_x[9] = (uintptr_t)td2;
-	td2->td_pcb->pcb_x[PCB_LR] = (uintptr_t)fork_trampoline;
+	td2->td_pcb->pcb_lr = (uintptr_t)fork_trampoline;
 	td2->td_pcb->pcb_sp = (uintptr_t)td2->td_frame;
 	td2->td_pcb->pcb_fpusaved = &td2->td_pcb->pcb_fpustate;
 	td2->td_pcb->pcb_vfpcpu = UINT_MAX;
@@ -173,7 +173,7 @@ cpu_copy_thread(struct thread *td, struct thread *td0)
 
 	td->td_pcb->pcb_x[8] = (uintptr_t)fork_return;
 	td->td_pcb->pcb_x[9] = (uintptr_t)td;
-	td->td_pcb->pcb_x[PCB_LR] = (uintptr_t)fork_trampoline;
+	td->td_pcb->pcb_lr = (uintptr_t)fork_trampoline;
 	td->td_pcb->pcb_sp = (uintptr_t)td->td_frame;
 	td->td_pcb->pcb_fpusaved = &td->td_pcb->pcb_fpustate;
 	td->td_pcb->pcb_vfpcpu = UINT_MAX;
@@ -265,7 +265,7 @@ cpu_fork_kthread_handler(struct thread *td, void (*func)(void *), void *arg)
 
 	td->td_pcb->pcb_x[8] = (uintptr_t)func;
 	td->td_pcb->pcb_x[9] = (uintptr_t)arg;
-	td->td_pcb->pcb_x[PCB_LR] = (uintptr_t)fork_trampoline;
+	td->td_pcb->pcb_lr = (uintptr_t)fork_trampoline;
 	td->td_pcb->pcb_sp = (uintptr_t)td->td_frame;
 	td->td_pcb->pcb_fpusaved = &td->td_pcb->pcb_fpustate;
 	td->td_pcb->pcb_vfpcpu = UINT_MAX;
diff --git a/sys/arm64/include/db_machdep.h b/sys/arm64/include/db_machdep.h
index 45d548c750bc..f2fd2a57a9c3 100644
--- a/sys/arm64/include/db_machdep.h
+++ b/sys/arm64/include/db_machdep.h
@@ -43,7 +43,7 @@
 typedef vm_offset_t	db_addr_t;
 typedef long		db_expr_t;
 
-#define	PC_REGS()	((db_addr_t)kdb_thrctx->pcb_pc)
+#define	PC_REGS()	((db_addr_t)kdb_thrctx->pcb_lr)
 
 #define	BKPT_INST	(0xd4200000)
 #define	BKPT_SIZE	(4)
diff --git a/sys/arm64/include/pcb.h b/sys/arm64/include/pcb.h
index 25c1a5a7aff4..012ed2c92741 100644
--- a/sys/arm64/include/pcb.h
+++ b/sys/arm64/include/pcb.h
@@ -36,10 +36,10 @@
 
 struct trapframe;
 
-#define	PCB_LR		30
 struct pcb {
-	uint64_t	pcb_x[31];
-	uint64_t	pcb_pc;
+	uint64_t	pcb_x[30];
+	uint64_t	pcb_lr;
+	uint64_t	_reserved;	/* Was pcb_pc */
 	/* These two need to be in order as we access them together */
 	uint64_t	pcb_sp;
 	uint64_t	pcb_tpidr_el0;