pcb.h: mark struct pcb to be preserved

There are programs that depend on this structure (e.g. kernel debuggers)
that breaks when the ABI changes.

Signed-off-by:	Minsoo Choo <minsoochoo0122@proton.me>
Reviewed by:	kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D55149

6 files changed, 39 insertions, 18 deletions

diff --git a/sys/amd64/include/pcb.h b/sys/amd64/include/pcb.h
index 27e1dce08ee1..5a1e8529ad8b 100644
--- a/sys/amd64/include/pcb.h
+++ b/sys/amd64/include/pcb.h
@@ -44,18 +44,19 @@
 
 #ifdef __amd64__
 /*
- * NB: The fields marked with (*) are used by kernel debuggers.  Their
- * ABI should be preserved.
+ * struct pcb is known to and used by kernel debuggers. Its layout must be kept
+ * stable. When adding extra fields that are accessed by kernel debuggers,
+ * debuggers should be backward compatible by using osreldate.
  */
 struct pcb {
-	register_t	pcb_r15;	/* (*) */
-	register_t	pcb_r14;	/* (*) */
-	register_t	pcb_r13;	/* (*) */
-	register_t	pcb_r12;	/* (*) */
-	register_t	pcb_rbp;	/* (*) */
-	register_t	pcb_rsp;	/* (*) */
-	register_t	pcb_rbx;	/* (*) */
-	register_t	pcb_rip;	/* (*) */
+	register_t	pcb_r15;
+	register_t	pcb_r14;
+	register_t	pcb_r13;
+	register_t	pcb_r12;
+	register_t	pcb_rbp;
+	register_t	pcb_rsp;
+	register_t	pcb_rbx;
+	register_t	pcb_rip;
 	register_t	pcb_fsbase;
 	register_t	pcb_gsbase;
 	register_t	pcb_kgsbase;
diff --git a/sys/arm/include/pcb.h b/sys/arm/include/pcb.h
index fd77544c22c1..810b1e665b19 100644
--- a/sys/arm/include/pcb.h
+++ b/sys/arm/include/pcb.h
@@ -42,6 +42,10 @@
 #include <machine/vfp.h>
 
 /*
+ * struct pcb is known to and used by kernel debuggers. Its layout must be kept
+ * stable. When adding extra fields that are accessed by kernel debuggers,
+ * debuggers should be backward compatible by using osreldate.
+ *
  * WARNING!
  * Keep pcb_regs first for faster access in switch.S
  */
diff --git a/sys/arm64/include/pcb.h b/sys/arm64/include/pcb.h
index c0feb1149cf5..9955f5b22714 100644
--- a/sys/arm64/include/pcb.h
+++ b/sys/arm64/include/pcb.h
@@ -46,6 +46,11 @@ struct trapframe;
 #define	PCB_FP		10
 #define	PCB_LR		11
 
+/*
+ * struct pcb is known to and used by kernel debuggers. Its layout must be kept
+ * stable. When adding extra fields that are accessed by kernel debuggers,
+ * debuggers should be backward compatible by using osreldate.
+ */
 struct pcb {
 	uint64_t	pcb_x[12];
 	/* These two need to be in order as we access them together */
diff --git a/sys/i386/include/pcb.h b/sys/i386/include/pcb.h
index 1b14efa425b5..1385bfeeef31 100644
--- a/sys/i386/include/pcb.h
+++ b/sys/i386/include/pcb.h
@@ -44,16 +44,17 @@
 #include <machine/npx.h>
 
 /*
- * NB: The fields marked with (*) are used by kernel debuggers.  Their
- * ABI should be preserved.
+ * struct pcb is known to and used by kernel debuggers. Its layout must be kept
+ * stable. When adding extra fields that are accessed by kernel debuggers,
+ * debuggers should be backward compatible by using osreldate.
  */
 struct pcb {
-	int	pcb_edi;	/* (*) */
-	int	pcb_esi;	/* (*) */
-	int	pcb_ebp;	/* (*) */
-	int	pcb_esp;	/* (*) */
-	int	pcb_ebx;	/* (*) */
-	int	pcb_eip;	/* (*) */
+	int	pcb_edi;
+	int	pcb_esi;
+	int	pcb_ebp;
+	int	pcb_esp;
+	int	pcb_ebx;
+	int	pcb_eip;
 	struct segment_descriptor pcb_fsd;
 	struct segment_descriptor pcb_gsd;
 	int	pcb_ds;
diff --git a/sys/powerpc/include/pcb.h b/sys/powerpc/include/pcb.h
index 0230cf78aba7..7ebd13dd0ed1 100644
--- a/sys/powerpc/include/pcb.h
+++ b/sys/powerpc/include/pcb.h
@@ -41,6 +41,11 @@
 #include <machine/setjmp.h>
 
 #ifndef _STANDALONE
+/*
+ * struct pcb is known to and used by kernel debuggers. Its layout must be kept
+ * stable. When adding extra fields that are accessed by kernel debuggers,
+ * debuggers should be backward compatible by using osreldate.
+ */
 struct pcb {
 	register_t	pcb_context[20];	/* non-volatile r12-r31 */
 	register_t	pcb_cr;			/* Condition register */
diff --git a/sys/riscv/include/pcb.h b/sys/riscv/include/pcb.h
index bb88516a1385..bcec4c3fd478 100644
--- a/sys/riscv/include/pcb.h
+++ b/sys/riscv/include/pcb.h
@@ -39,6 +39,11 @@
 
 struct trapframe;
 
+/*
+ * struct pcb is known to and used by kernel debuggers. Its layout must be kept
+ * stable. When adding extra fields that are accessed by kernel debuggers,
+ * debuggers should be backward compatible by using osreldate.
+ */
 struct pcb {
 	uint64_t	pcb_ra;		/* Return address */
 	uint64_t	pcb_sp;		/* Stack pointer */