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 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.

Jan


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