Re: [Qemu-devel] [PATCH RFC] ps2: set the keybord output buffer size as the same as kernel

[Gerd Hoffmann]

On Di, 2014-04-22 at 08:16 +0000, Gonglei (Arei) wrote:
> > 
> > > diff --git a/hw/input/ps2.c b/hw/input/ps2.c
> > > index 3412079..a754fef 100644
> > > --- a/hw/input/ps2.c
> > > +++ b/hw/input/ps2.c
> > > @@ -71,7 +71,7 @@
> > >  #define MOUSE_STATUS_ENABLED    0x20
> > >  #define MOUSE_STATUS_SCALE21    0x10
> > >
> > > -#define PS2_QUEUE_SIZE 256
> > > +#define PS2_QUEUE_SIZE 16     /* Keyboard output buffer size */
> > >
> > >  typedef struct {
> > >      uint8_t data[PS2_QUEUE_SIZE];
> > 
> > This changes ps2 vmstate and breaks live migration.
> > 
> Good catch, Gerd.
> I got the information in the destination of live migration:
> Unknown savevm section type 24
> load of migration failed
> 
> I'm not familiar with the situation of cross-version live migration, could 
> you give me 
> some guide ? Thanks.

Keep the data array 256 bytes long, best with a comment that
compatibility with older qemu versions requires this.

Also the post_load function must handle the case that rptr, wptr & count
variables have values which used to be valid for the older qemu versions
but are not valid any more with the smaller queue.  In the (unlikely)
case that count is larger than 16 the best you can do is probably simply
throw away the queue.  16 and less queue elements you can move to the
start of the data array (so they are within the 16 bytes still used
after your patch is merged) and adjust rptr+wptr accordingly.

Cc'ing Juan for additional insights.

HTH,
  Gerd