powerpc64: give each CPU its own KDB trap stack

Kernel mode program, trace, breakpoint and machine check exceptions go
through dbtrap, which saves the registers on trapstk and, if KDB doesn't
want the trap, restores them from there and continues with the normal
trap handler. There was just one trapstk for all CPUs, so when two of
them trapped at the same time they overwrote each other's frame and one
of them carried on with the other's registers. DTrace fbt probes are
these traps, so a probe on a function that runs on several CPUs at once
took the kernel down within seconds.

Keep the stack in the pcpu instead. The boot CPU gets a static one,
since it's set up before malloc works, the others allocate theirs in
cpu_pcpu_init() like the Book-E critical interrupt stack.

Approved by:	jhibbits
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D60362

sys/powerpc/aim/aim_machdep.c

@@ -529,6 +529,20 @@ kdb_cpu_set_singlestep(void)
 	kdb_frame->srr1 |= PSL_SE;
 }
 
+#if defined(__powerpc64__) && defined(KDB)
+/* Stack used by the KDB trap handler (dbtrap) on each CPU. */
+#define	AIM_DBSTACK_SIZE	8192
+
+/*
+ * Stack for the KDB trap handler on the boot CPU.  It is static because the
+ * boot CPU is initialised long before malloc works; APs reach cpu_pcpu_init()
+ * at SI_SUB_CPU and allocate theirs.  Sizing a MAXCPU array would reserve
+ * megabytes for CPUs that do not exist.
+ */
+static char aim_boot_dbstack[AIM_DBSTACK_SIZE] __aligned(16);
+static bool aim_boot_dbstack_used;
+#endif
+
 /*
  * Initialise a struct pcpu.
  */
@@ -536,8 +550,21 @@ void
 cpu_pcpu_init(struct pcpu *pcpu, int cpuid, size_t sz)
 {
 #ifdef __powerpc64__
+#ifdef KDB
+	char *dbstack;
+#endif
+
 /* Copy the SLB contents from the current CPU */
 memcpy(pcpu->pc_aim.slb, PCPU_GET(aim.slb), sizeof(pcpu->pc_aim.slb));
+
+#ifdef KDB
+	if (!aim_boot_dbstack_used) {
+		aim_boot_dbstack_used = true;
+		dbstack = aim_boot_dbstack;
+	} else
+		dbstack = malloc(AIM_DBSTACK_SIZE, M_DEVBUF, M_WAITOK);
+	pcpu->pc_aim.dbstack = dbstack + AIM_DBSTACK_SIZE;
+#endif
 #endif
 }
 

sys/powerpc/aim/locore64.S

@@ -62,13 +62,6 @@ GLOBAL(tmpstk)
 TOC_ENTRY(tmpstk)
 TOC_ENTRY(can_wakeup)
 
-#ifdef KDB
-#define TRAPSTKSZ       8192            /* 8k trap stack */
-GLOBAL(trapstk)
-        .space        TRAPSTKSZ
-TOC_ENTRY(trapstk)
-#endif
-
 
 /*
  * Entry point for bootloaders that do not fully implement ELF and start

sys/powerpc/aim/trap_subr64.S

@@ -920,10 +920,9 @@ dbtrap:
 	andi.	%r1,%r1,0xff00
 	mtsprg3	%r1
 
-	GET_TOCBASE(%r1)			/* get new SP */
-	addis	%r1,%r1,TOC_REF(trapstk)@ha
-	ld	%r1,TOC_REF(trapstk)@l(%r1)
-	addi	%r1,%r1,(TRAPSTKSZ-48)
+	GET_CPUINFO(%r1)			/* this CPU's trap stack */
+	ld	%r1,PC_AIM_DBSTACK(%r1)
+	addi	%r1,%r1,-48
 
 	FRAME_SETUP(PC_DBSAVE)
 /* Call C trap code: */

sys/powerpc/include/pcpu.h

@@ -69,7 +69,8 @@ struct pvo_entry;
 	struct pvo_entry *qmap_pvo;					\
 	struct mtx	qmap_lock;					\
 	uint64_t	opal_hmi_flags;					\
-	char		__pad[1337];
+	void		*dbstack;		/* KDB trap stack top */	\
+	char		__pad[1329];
 
 #ifdef __powerpc64__
 #define PCPU_MD_AIM_FIELDS	PCPU_MD_AIM64_FIELDS

sys/powerpc/powerpc/genassym.c

@@ -114,6 +114,7 @@ ASSYM(PC_KERNSLB, offsetof(struct pcpu, pc_aim.slb));
 ASSYM(PC_USERSLB, offsetof(struct pcpu, pc_aim.userslb));
 ASSYM(PC_SLBSAVE, offsetof(struct pcpu, pc_aim.slbsave));
 ASSYM(PC_SLBSTACK, offsetof(struct pcpu, pc_aim.slbstack));
+ASSYM(PC_AIM_DBSTACK, offsetof(struct pcpu, pc_aim.dbstack));
 ASSYM(USER_SLB_SLOT, USER_SLB_SLOT);
 ASSYM(USER_SLB_SLBE, USER_SLB_SLBE);
 ASSYM(SEGMENT_MASK, SEGMENT_MASK);