lkml.org 
[lkml]   [2018]   [Sep]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: How to handle PTE tables with non contiguous entries ?
From
Date


Le 17/09/2018 à 11:03, Aneesh Kumar K.V a écrit :
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>
>> Hi,
>>
>> I'm having a hard time figuring out the best way to handle the following
>> situation:
>>
>> On the powerpc8xx, handling 16k size pages requires to have page tables
>> with 4 identical entries.
>
> I assume that hugetlb page size? If so isn't that similar to FSL hugetlb
> page table layout?

No, it is not for 16k hugepage size with a standard page size of 4k.

Here I'm trying to handle the case of CONFIG_PPC_16K_PAGES.
As of today, it is implemented by using the standard Linux page layout,
ie one PTE entry for each 16k page. This forbids the use the 8xx HW
assistance.

>
>>
>> Initially I was thinking about handling this by simply modifying
>> pte_index() which changing pte_t type in order to have one entry every
>> 16 bytes, then replicate the PTE value at *ptep, *ptep+1,*ptep+2 and
>> *ptep+3 both in set_pte_at() and pte_update().
>>
>> However, this doesn't work because many many places in the mm core part
>> of the kernel use loops on ptep with single ptep++ increment.
>>
>> Therefore did it with the following hack:
>>
>> /* PTE level */
>> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
>> +typedef struct { pte_basic_t pte, pte1, pte2, pte3; } pte_t;
>> +#else
>> typedef struct { pte_basic_t pte; } pte_t;
>> +#endif
>>
>> @@ -181,7 +192,13 @@ static inline unsigned long pte_update(pte_t *p,
>> : "cc" );
>> #else /* PTE_ATOMIC_UPDATES */
>> unsigned long old = pte_val(*p);
>> - *p = __pte((old & ~clr) | set);
>> + unsigned long new = (old & ~clr) | set;
>> +
>> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
>> + p->pte = p->pte1 = p->pte2 = p->pte3 = new;
>> +#else
>> + *p = __pte(new);
>> +#endif
>> #endif /* !PTE_ATOMIC_UPDATES */
>>
>> #ifdef CONFIG_44x
>>
>>
>> @@ -161,7 +161,11 @@ static inline void __set_pte_at(struct mm_struct
>> *mm, unsigned long addr,
>> /* Anything else just stores the PTE normally. That covers all
>> 64-bit
>> * cases, and 32-bit non-hash with 32-bit PTEs.
>> */
>> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
>> + ptep->pte = ptep->pte1 = ptep->pte2 = ptep->pte3 = pte_val(pte);
>> +#else
>> *ptep = pte;
>> +#endif
>>
>>
>>
>> But I'm not too happy with it as it means pte_t is not a single type
>> anymore so passing it from one function to the other is quite heavy.
>>
>>
>> Would someone have an idea of an elegent way to handle that ?
>>
>> Thanks
>> Christophe
>
> Why would pte_update bother about updating all the 4 entries?. Can you
> help me understand the issue?

Because the 8xx HW assistance expects 4 identical entries for each 16k
page, so everytime a PTE is updated the 4 entries have to be updated.

Christophe

\
 
 \ /
  Last update: 2018-09-17 11:49    [W:0.073 / U:1.260 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site