Re: [Qemu-devel] Re: [PATCH] hpet: Clean up initial hpet counter

[Gleb Natapov]

On Wed, Jun 16, 2010 at 06:00:56PM +0200, Jan Kiszka wrote:
> Gleb Natapov wrote:
> > On Wed, Jun 16, 2010 at 12:35:16PM +0300, Gleb Natapov wrote:
> >> On Wed, Jun 16, 2010 at 11:33:13AM +0200, Jan Kiszka wrote:
> >>> Gleb Natapov wrote:
> >>>> On Wed, Jun 16, 2010 at 09:57:35AM +0200, Jan Kiszka wrote:
> >>>>> Gleb Natapov wrote:
> >>>>>> On Wed, Jun 16, 2010 at 09:51:14AM +0200, Jan Kiszka wrote:
> >>>>>>> Gleb Natapov wrote:
> >>>>>>>> On Wed, Jun 16, 2010 at 09:03:01AM +0200, Jan Kiszka wrote:
> >>>>>>>>> Gleb Natapov wrote:
> >>>>>>>>>> On Wed, Jun 16, 2010 at 12:40:28AM +0200, Jan Kiszka wrote:
> >>>>>>>>>>> From: Jan Kiszka <address@hidden>
> >>>>>>>>>>>
> >>>>>>>>>>> There is no need starting with the special value for 
> >>>>>>>>>>> hpet_cfg.count.
> >>>>>>>>>>> Either Seabios is aware of the new firmware interface and properly
> >>>>>>>>>>> interprets the counter or it simply ignores it anyway.
> >>>>>>>>>>>
> >>>>>>>>>> I want seabios to be able to distinguish between old qemu and new 
> >>>>>>>>>> one.
> >>>>>>>>> I see now. But isn't it a good chance to introduce a proper generic
> >>>>>>>>> interface for exploring supported fw-cfg keys?
> >>>>>>>>>
> >>>>>>>> Having such interface would be nice. Pity we haven't introduced it 
> >>>>>>>> from
> >>>>>>>> the start. If we do it now seabios will have to find out somehow that
> >>>>>>>> qemu support such interface. Chicken and egg ;)
> >>>>>>> That is easy: Add a key the describes the highest supported key value
> >>>>>>> (looks like this is monotonously increasing). Older qemu versions will
> >>>>>>> return 0.
> >>>>>>>
> >>>>>> That will not support holes in key space, and our key space is already
> >>>>>> sparse.
> >>>>> Then add a service to obtain a bitmap of supported keys. If that bitmap
> >>>>> is empty...
> >>>>>
> >>>> Bitmap will be 2k long. We can add read capability to control port. To
> >>>> check if key is present you select it (write its value to control port)
> >>>> and then read control port back. If values is non-zero the key is valid.
> >>>> But how to detect qemu that does not support that?
> >>> Isn't there some key that was always there and will always be?
> >>>
> >> FW_CFG_SIGNATURE
> >>
> > So any ideas? Or did I misunderstood your hint? ;)
> 
> I thought you found the answer yourself:
> 
> Seabios could select FW_CFG_SIGNATURE and then perform a read-back on
> the control register. Older QEMUs will return -1, versions that support
> the read-back 0. Problem solved, no?
> 
AFAIK QEMU returns 0 if io read was done from non-used port or mmio
address, but can we rely on this? If we can then problem solved, if
we can't then no.

--
                        Gleb.