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 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".

Undefined IO ports return -1, undefined (/wrt read access) MMIO 0. So
you need to select a key that is different from both.

> Can you write pseudo logic of how you think it
> all should work?

The firmware should do this:

write(CTL_BASE, FW_CFG_ID);
if (read(CTL_BASE) != FW_CFG_ID)
	deal_with_old_qemu();
else
	check_for_supported_keys();

Jan


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