Re: [Qemu-devel] [PATCH] ps2: take exact use of ps2 buffer

[Yang Hongyang]

On Thu, Jun 2, 2016 at 6:05 PM, Gerd Hoffmann <address@hidden> wrote:

> On Do, 2016-06-02 at 17:19 +0800, Yang Hongyang wrote:
> > Hi Gerd,
> >   Thanks for reply!
> >
> >
> > On Thu, Jun 2, 2016 at 4:37 PM, Gerd Hoffmann <address@hidden>
> > wrote:
> >         On Do, 2016-06-02 at 14:05 +0800, Yang Hongyang wrote:
> >         > According to PS/2 Mouse/Keyboard Protocol, the keyboard
> >         output buffer
> >         > size is 16 bytes, but we only use 15 bytes actually, this
> >         causes some
> >         > problem, for example, if I submit "123456789" in a bunch
> >         through VNC,
> >         > the actual result will be "12345678888888888...", because
> >         the 16th key
> >         > event which is "8" key up is dropped.
> >
> >         What if you try a 10-char string next?
> >
> >
> > Actually I've tested this patch, I submit multiple 10-char string,
> > things work
> > as expected, it only takes the first 8-char.
>
> That is pure luck.  It could have taken the first 9 chars in case the
> guest manages to take one out of the queue while you are filling the
> queue.  It's a race and nothing you can depend on.


> > The poin is not about to make it work with more then 8 string, it is
> > to
> > make it competible with the protocol, which is a 16-bytes
> > buffer, apparantly
> > we are not following this and which do cause the problem.
>
> There is no such protocol.
>
> On physical hardware things are working fine because you simply can't
> type fast enough to overflow the buffer.  On virtual hardware you can if
> you script keyboard input, thereby breaking things, simply because ps/2
> wasn't designed for that.  Whenever the buffer is 15 bytes or 16 bytes
> or something else (with usb-kbd) doesn't matter, the underlying issue is
> always the same.  And the solution is to apply a speed limit to kbd
> input.
>

Thanks for the explaination, I got your point, by add the speed limit to kbd
input, even when I send a bulk kbd event through vnc, the kbd will have
time to
read the data. before this, vnc will fulfill the buffer until qemu have
time to to schedule a kbd_read_data call. I've tested your patch, and it
solved my problem, thank you.


>
> > This chunk of code originally uses full QUEUE_SIZE buffer, and in
> > this commit 2858ab09e6f708e3, it changes the behaviour implicitly.
>
> It's a workaround for older linux kernels.  They misbehave in case they
> find 16 chars in the ps2 buffer (this is something which simply doesn't
> happen on real hardware).
>
> cheers,
>   Gerd
>
>
>
>


-- 
Thanks,
Yang