Determine the operand/address size of %cs in a new function

db_segsize().

Use db_segsize() to set the default operand/address size for
disassembling.  Allow overriding this with the "alternate" display
format /I.  The API of db_disasm() should be debooleanized to pass a
more general request (amd64 needs overrides to sizes of 16, 32, and
64, but this commit doesn't implement anything for amd64 since much
larger changes are needed to restore the amd64 disassmbler's support
for non-default sizes).

Fix db_print_loc_and_inst() to ask for the normal format and not the
alternate in normal operation.

This is most useful for vm86 mode, but also works for 16-bit protected
mode.

Use db_segsize() to avoid trying to print a garbage stack trace if %cs
is 16 bits.  Print something like the stack trace termination message
for a trap boundary instead.

Document that the alternate format is now useful on i386.

share/man/man4/ddb.4

@@ -264,7 +264,9 @@ The location is also displayed in hex at the beginning of each line.
 display as an instruction
 .It Cm I
 display as an instruction with possible alternate formats depending on the
-machine, but none of the supported architectures have an alternate format
+machine.
+On i386, this selects the alternate format for the instruction decoding
+(16 bits in a 32-bit code segment and vice versa).
 .It Cm S
 display a symbol name for the pointer stored at the address
 .El

sys/ddb/db_examine.c

@@ -241,7 +241,7 @@ db_print_loc_and_inst(db_addr_t loc)
 	db_printsym(loc, DB_STGY_PROC);
 	if (db_search_symbol(loc, DB_STGY_PROC, &off) != C_DB_SYM_NULL) {
 		db_printf(":\t");
-		(void)db_disasm(loc, true);
+		(void)db_disasm(loc, false);
 	}
 }
 

sys/i386/i386/db_disasm.c

@@ -31,6 +31,7 @@ __FBSDID("$FreeBSD$");
  * Instruction disassembler.
  */
 #include <sys/param.h>
+#include <sys/kdb.h>
 
 #include <ddb/ddb.h>
 #include <ddb/db_access.h>
@@ -1168,9 +1169,17 @@ db_disasm(db_addr_t loc, bool altfmt)
 	int	len;
 	struct i_addr	address;
 
+	if (db_segsize(kdb_frame) == 16)
+	   altfmt = !altfmt;
 	get_value_inc(inst, loc, 1, FALSE);
+	if (altfmt) {
+	    short_addr = TRUE;
+	    size = WORD;
+	}
+	else {
 	    short_addr = FALSE;
 	    size = LONG;
+	}
 	seg = NULL;
 
 	/*

sys/i386/i386/db_interface.c

@@ -135,6 +135,30 @@ db_write_bytes(vm_offset_t addr, size_t size, char *data)
 	return (ret);
 }
 
+int
+db_segsize(struct trapframe *tfp)
+{
+	struct proc_ldt *plp;
+	struct segment_descriptor *sdp;
+	int sel;
+
+	if (tfp == NULL)
+	    return (32);
+	if (tfp->tf_eflags & PSL_VM)
+	    return (16);
+	sel = tfp->tf_cs & 0xffff;
+	if (sel == GSEL(GCODE_SEL, SEL_KPL))
+	    return (32);
+	/* Rare cases follow.  User mode cases are currently unreachable. */
+	if (ISLDT(sel)) {
+	    plp = curthread->td_proc->p_md.md_ldt;
+	    sdp = (plp != NULL) ? &plp->ldt_sd : &ldt[0].sd;
+	} else {
+	    sdp = &gdt[PCPU_GET(cpuid) * NGDT].sd;
+	}
+	return (sdp[IDXSEL(sel)].sd_def32 == 0 ? 16 : 32);
+}
+
 void
 db_show_mdpcpu(struct pcpu *pc)
 {

sys/i386/i386/db_trace.c

@@ -421,6 +421,17 @@ db_backtrace(struct thread *td, struct trapframe *tf, struct i386_frame *frame,
 	int instr, narg;
 	boolean_t first;
 
+	if (db_segsize(tf) == 16) {
+		db_printf(
+"--- 16-bit%s, cs:eip = %#x:%#x, ss:esp = %#x:%#x, ebp = %#x, tf = %p ---\n",
+		    (tf->tf_eflags & PSL_VM) ? " (vm86)" : "",
+		    tf->tf_cs, tf->tf_eip,
+		    TF_HAS_STACKREGS(tf) ? tf->tf_ss : rss(),
+		    TF_HAS_STACKREGS(tf) ? tf->tf_esp : (intptr_t)&tf->tf_esp,
+		    tf->tf_ebp, tf);
+		return (0);
+	}
+
 	/*
 	 * If an indirect call via an invalid pointer caused a trap,
 	 * %pc contains the invalid address while the return address

sys/i386/include/db_machdep.h

@@ -98,4 +98,6 @@ do {						\
 #define	DB_SMALL_VALUE_MAX	0x7fffffff
 #define	DB_SMALL_VALUE_MIN	(-0x400001)
 
+int	db_segsize(struct trapframe *tfp);
+
 #endif /* !_MACHINE_DB_MACHDEP_H_ */