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

[Gleb Natapov <gleb@redhat.com>]

On Thu, Jun 17, 2010 at 09:17:51AM +0200, Jan Kiszka wrote:
> Gleb Natapov wrote:
> > 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 <jan.kiszka@siemens.com>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> 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.
> 
> It works for IO-based fw-cfg, but not for MMIO-based. So the firmware
> should probably pick a non-zero key for this check, e.g. FW_CFG_ID.
> 
Sorry, I lost you here. What "works for IO-based fw-cfg, but not for
MMIO-based". Can you write pseudo logic of how you think it
all should work?

--
			Gleb.