Runtime check for PAE option on BSD 6+ i386

[Philip Soeberg]

On 03-05-2011 19:43, Kostik Belousov wrote:
> On Tue, May 03, 2011 at 11:44:32AM -0400, John Baldwin wrote:
>> On Tuesday, May 03, 2011 9:10:26 am Philip Soeberg wrote:
>>> Hi fellow FreeBSD hackers,
>>>
>>> I've been using the following poor-man's approach in my driver init for
>>> ages in an attempt at detecting PAE option on BSD 6 (or greater) i386
>>> kernels, as I depend on dmabus(9) but provide a loadable kernel module only.
>>>
>>>   >>>
>>>     if (sizeof(void*) == 4) {
>>>       if (((uint64_t)(cnt.v_page_count * cnt.v_page_size) / 1073741824)
>>>   >= 4) {
>>>         printf("FreeBSD i386 detected with PAE option enabled. FreeBSD
>>> PAE type\n");
>>>         printf("kernels does not support loadable modules which use DMA.
>>> Please\n");
>>>         printf("reconfigure your kernel for non-PAE or switch to amd64
>>> kernel.\n");
>>>         return EFAULT;
>>>       }
>>>     }
>>> <<<
>> Hmmm, even this isn't really accurate as some folks may choose to enable PAE
>> even with<  4GB to get PG_NX functionality.
>>
>>> afaik there's a sysctl method of checking this per BSD7 (or is it 8?),
>>> but what about BSD6? Any hints on how I can runtime detect the above?
>> Definitely a kern.features.pae sysctl in 7.  I don't see anything similar in
>> 6.
> Read %cr4 and test the bit there.
Ha, as always there exist a simple elegant solution.. thanks, that 
method had escaped me.