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Subject[PATCH] KDB: Fix usability issues relating to the 'enter' key.
This fixes the following problems:
1) Typematic-repeat of 'enter' gives warning message.
2) Use of 'keypad enter' gives warning message.
3) Lag on the order of seconds between "break" and "make" when
expecting the enter "break" code. Seen under virtualized
environments such as VMware ESX.

1) Holding down 'enter' will not set a repeating sequence
of 0x1c(make)-0x9c(make), but a repeating sequence
of make codes, followed by one break code when the key
is released. Thus, it's wrong to expect the break code
after seeing the 'enter' make code.
2) Keypad enter generates different make/break, namely
0xe0 0x1c and 0xe0 0x9c. The 'generic' logic handles
the 0xe0 escape already, but the special 'enter' logic
always expects '0x9c' and not '0xe0 0x9c', so you get
a warning message, again.
3) When expecting the 'enter' break code, the code polls
the status register in a tight loop, like so -
> while ((inb(KBD_STATUS_REG) & KBD_STAT_OBF) == 0);

However, it really should do something like -
> while ((inb(KBD_STATUS_REG) & KBD_STAT_OBF) == 0)
> cpu_relax(); /* pause */

Basically, it's a common optimization to have a fast
path for accessing often accessed and slow changing I/O
in a virtualized environment. The tight spinning in KDB
seems to run against the logic by ESX keyboard virtualization
code to detect when the fast path or the slow path should
be used to satisfy the keyboard status read, leading to
multi-second timeouts before the 'real' status comes through.
Without knowing ESX internals, it's hard to say if this is
an ESX bug or not, but letting the VM be explicitely descheduled
seems to resolve the problem. I've seen something similar with
shared MMIO buffers with VMs on Hyper-V.

Anyway, given (3), (2) and (1), we might as well blow away the
entire special casing for 'enter'. The break codes will already
be handled correctly, and we get rid of the bugs with repeat
enters and keypad enter key. And of course, there is no
need to AND with 0x7f when checking for 'enter', because we'll
never ever get to this code with a break code (checked for much

I tried to figure out the history behind the 'enter' key special
casing code, and it seems to have come from whatever the original
KDB patch was. Perhaps someone can chime in.

Tested on ESX 5.0 and QEMU.

Signed-off-by: Andrei Warkentin <>
kernel/debug/kdb/kdb_keyboard.c | 28 +---------------------------
1 files changed, 1 insertions(+), 27 deletions(-)

diff --git a/kernel/debug/kdb/kdb_keyboard.c b/kernel/debug/kdb/kdb_keyboard.c
index 4bca634..ed4a2f9 100644
--- a/kernel/debug/kdb/kdb_keyboard.c
+++ b/kernel/debug/kdb/kdb_keyboard.c
@@ -178,34 +178,8 @@ int kdb_get_kbd_char(void)
return -1; /* ignore unprintables */

- if ((scancode & 0x7f) == 0x1c) {
- /*
- * enter key. All done. Absorb the release scancode.
- */
- while ((inb(KBD_STATUS_REG) & KBD_STAT_OBF) == 0)
- ;
- /*
- * Fetch the scancode
- */
- scancode = inb(KBD_DATA_REG);
- scanstatus = inb(KBD_STATUS_REG);
- while (scanstatus & KBD_STAT_MOUSE_OBF) {
- scancode = inb(KBD_DATA_REG);
- scanstatus = inb(KBD_STATUS_REG);
- }
- if (scancode != 0x9c) {
- /*
- * Wasn't an enter-release, why not?
- */
- kdb_printf("kdb: expected enter got 0x%x status 0x%x\n",
- scancode, scanstatus);
- }
+ if (scancode == 0x1c)
return 13;
- }

return keychar & 0xff;

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