Re: [Qemu-devel] [PATCH v2 1/6] qerror: add MAX_KEYCODES 16

[Amos Kong <akong@redhat.com>]

On 18/06/12 23:30, Amos Kong wrote:
> On 06/15/2012 09:35 PM, Luiz Capitulino wrote:
>> On Fri, 15 Jun 2012 09:57:49 +0200
>> Gerd Hoffmann<kraxel@redhat.com>  wrote:
>>
>>>    Hi,
>>>
>>>>> It seems we need to notice user when inputted keys are more than 16.
>>>>
>>>> Hi Gerd,
>>>>
>>>> When I use 'sendkey' command to send key-series to guest, some keyboard
>>>> events will be send. There is a limitation (16) that was introduced by this
>>>> old commit c8256f9d (without description). Do you know the reason?
>>>
>>> Probably hardware limitation, ps/2 keyboards can buffer up to 16 keys IIRC.
>>
>> Then the perfect thing to do would be to drop the MAX_KEYCODES check from
>> the sendkey command and move bounds checking down to the device emulation code.
>>
>>
>> However, this will require a bit of code churn if we do it for all devices,
>> and won't buy us much, as the most likely reason for the error is a client/user
>> trying to send too many keys in parallel to the guest, right?
>
> Agree, we can notice in stderr when the redundant keys are ignored as hid.
>
>
> #define QUEUE_LENGTH    16 /* should be enough for a triple-click */
>
> static void hid_keyboard_event(void *opaque, int keycode)
> {
>      ...
>      if (hs->n == QUEUE_LENGTH) {
>          fprintf(stderr, "usb-kbd: warning: key event queue full\n");
>          return;
>      }


I dropped the limitation in sendkey command,
and didn't change current ps2.c, executed some
tests in different environments.

environment             max inputted key number
------------            -----------------------
win7 notepad            100
rhel6 grub              15
rhel6 pxe               15
rhel6 login window      10
rhel6 vim               16
rhel6 terminal(init 3)  200


It seems original 256 queue limitation in ps2.c is fine.
I would only drop limitation(16) in old sendkey command,
it's secure.


>> If this is right, then I think that the best thing to do would be to drop the
>> MAX_KEYCODES check from the sendkey command and document that devices can drop
>> keys if too many of them are sent in parallel or too fast (we can mention ps/2
>> as an example of a 16 bytes limit).
>>
>>>
>>> Likewise the usb hid devices can buffer up to 16 events.  In that case
>>> it is just a qemu implementation detail and not a property of the
>>> hardware we are emulating, so it can be changed.  Not trivially though
>>> as the buffer is part of the migration data, so it is more work that
>>> just changing a #define.


-- 
			Amos.