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

[Jan Kiszka <jan.kiszka@web.de>]

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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?

Jan


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