Le 18/09/2018 à 13:47, Aneesh Kumar K.V a écrit :
Christophe LEROY <christophe.leroy@xxxxxx> writes:
Le 17/09/2018 à 11:03, Aneesh Kumar K.V a écrit :
Christophe Leroy <christophe.leroy@xxxxxx> 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.
What you suggested in the original mail is what matches that best isn't it?
That is a linux pte update involves updating 4 slot. Hence a linux pte
consist of 4 unsigned long?
Yes indeed.
It seems hopeless to avoid carrying the 4 longs from one function to the
other allthough that's four times the same thing.
Christophe
