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diff --git a/share/man/man4/ddb.4 b/share/man/man4/ddb.4
--- a/share/man/man4/ddb.4
+++ b/share/man/man4/ddb.4
@@ -955,14 +955,10 @@
Display the register set.
If the
.Cm u
-modifier is specified, it displays user registers instead of
-kernel registers or the currently saved one.
+modifier is specified, the register contents of the thread's previous
+trapframe are displayed instead.
+Usually, this corresponds to the saved state from userspace.
.Pp
-.Sy Warning :
-The support of the
-.Cm u
-modifier depends on the machine.
-If not supported, incorrect information will be displayed.
.\"
.Pp
.It Ic show Cm rman Ar addr
diff --git a/sys/ddb/db_print.c b/sys/ddb/db_print.c
--- a/sys/ddb/db_print.c
+++ b/sys/ddb/db_print.c
@@ -49,12 +49,32 @@
#include <ddb/db_sym.h>
void
-db_show_regs(db_expr_t _1, bool _2, db_expr_t _3, char *_4)
+db_show_regs(db_expr_t _1, bool _2, db_expr_t _3, char *modif)
{
+ struct trapframe *oldtf;
struct db_variable *regp;
db_expr_t value, offset;
const char *name;
+ /*
+ * The 'u' modifier instructs us to print the previous trapframe, most
+ * often containing state from userspace. This is done by temporarily
+ * switching out kdb_frame.
+ *
+ * NB: curthread is used instead of kdb_thread, so that behaviour is
+ * consistent with regular `show registers`, which always prints
+ * curthread's trapframe.
+ */
+ oldtf = kdb_frame;
+ if (modif[0] == 'u') {
+ if (curthread->td_frame == NULL ||
+ curthread->td_frame == oldtf) {
+ db_printf("previous trapframe unavailable");
+ return;
+ }
+ kdb_frame = curthread->td_frame;
+ }
+
for (regp = db_regs; regp < db_eregs; regp++) {
if (!db_read_variable(regp, &value))
continue;
@@ -70,4 +90,6 @@
db_printf("\n");
}
db_print_loc_and_inst(PC_REGS());
+
+ kdb_frame = oldtf;
}

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