Re: [PATCH] KVM: PPC: E500: Support hugetlbfs

[Scott Wood <scottwood@freescale.com>]

On 09/24/2011 02:44 AM, Alexander Graf wrote:
> On 20.09.2011, at 19:54, Scott Wood wrote:
>> On 09/19/2011 06:35 PM, Alexander Graf wrote:
>>> +			asm (PPC_CNTLZL "%0,%1" : "=r" (lz) : "r" (psize));
>>> +			tsize = min(21 - lz, tsize);
>>
>> No need to open-code the cntlz and subtract-from-21:
>>
>> 	tsize = min(ilog2(psize) - 10, tsize);
>>
>> 	/*
>> 	 * e500 doesn't implement the lowest tsize bit,
>> 	 * or 1K pages.
>> 	 */
>> 	tsize = max(BOOK3E_PAGESZ_4K, tsize & ~1);
>>
>> There's still an open-coded subtraction of 10, but that relates more
>> straightforwardly to the definition of tsize (and could be factored out
>> into a size-to-tsize function).
> 
> Yeah, no need to micro-optimized those few bits. The reason I used the asm statement was that I copied the hugetlbfs code which does it that way :).

The "21 - lz" thing is broken on 64-bit, FWIW.  It works by chance in
the hugetlbfs code (and in settlbcam) because it results in tsize being
too low by 32, which only affects bits that subsequently get masked off.

>>> 		}
>>>
>>> 		up_read(&current->mm->mmap_sem);
>>> 	}
>>>
>>> 	if (likely(!pfnmap)) {
>>> +		unsigned long tsize_pages = 1 << (tsize - 2);
>>
>> 1 << (tsize + 10 - PAGE_SHIFT);
> 
> Are we getting variable page sizes anytime soon? Will change it nevertheless, just curious :).

Nothing imminent on our chips AFAIK, but there's some 64K page support
in Linux for IBM's book3e chips, and it's not nice to hardcode
(especially in a hidden way) regardless.

-Scott