powerpc/8xx: Manage 512k huge pages as standard pages.

At the time being, 512k huge pages are handled through hugepd page
tables. The PMD entry is flagged as a hugepd pointer and it
means that only 512k hugepages can be managed in that 4M block.
However, the hugepd table has the same size as a normal page
table, and 512k entries can therefore be nested with normal pages.

On the 8xx, TLB loading is performed by software and allthough the
page tables are organised to match the L1 and L2 level defined by
the HW, all TLB entries have both L1 and L2 independent entries.
It means that even if two TLB entries are associated with the same
PMD entry, they can be loaded with different values in L1 part.

The L1 entry contains the page size (PS field):
- 00 for 4k and 16 pages
- 01 for 512k pages
- 11 for 8M pages

By adding a flag for hugepages in the PTE (_PAGE_HUGE) and copying it
into the lower bit of PS, we can then manage 512k pages with normal
page tables:
- PMD entry has PS=11 for 8M pages
- PMD entry has PS=00 for other pages.

As a PMD entry covers 4M areas, a PMD will either point to a hugepd
table having a single entry to an 8M page, or the PMD will point to
a standard page table which will have either entries to 4k or 16k or
512k pages. For 512k pages, as the L1 entry will not know it is a
512k page before the PTE is read, there will be 128 entries in the
PTE as if it was 4k pages. But when loading the TLB, it will be
flagged as a 512k page.

Note that we can't use pmd_ptr() in asm/nohash/32/pgtable.h because
it is not defined yet.

In ITLB miss, we keep the possibility to opt it out as when kernel
text is pinned and no user hugepages are used, we can save several
instruction by not using r11.

In DTLB miss, that's just one instruction so it's not worth bothering
with it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/002819e8e166bf81d24b24782d98de7c40905d8f.1589866984.git.christophe.leroy@csgroup.eu

1 files changed, 5 insertions, 7 deletions

diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
index adad8baadcf5..423465b10c82 100644
--- a/arch/powerpc/kernel/head_8xx.S
+++ b/arch/powerpc/kernel/head_8xx.S
@@ -239,7 +239,6 @@ InstructionTLBMiss:
 #endif
 #ifdef CONFIG_HUGETLBFS
 	lwz	r11, (swapper_pg_dir-PAGE_OFFSET)@l(r10)	/* Get level 1 entry */
-	mtspr	SPRN_MI_TWC, r11	/* Set segment attributes */
 	mtspr	SPRN_MD_TWC, r11
 #else
 	lwz	r10, (swapper_pg_dir-PAGE_OFFSET)@l(r10)	/* Get level 1 entry */
@@ -248,6 +247,10 @@ InstructionTLBMiss:
 #endif
 	mfspr	r10, SPRN_MD_TWC
 	lwz	r10, 0(r10)	/* Get the pte */
+#ifdef CONFIG_HUGETLBFS
+	rlwimi	r11, r10, 32 - 9, _PMD_PAGE_512K
+	mtspr	SPRN_MI_TWC, r11
+#endif
 #ifdef CONFIG_SWAP
 	rlwinm	r11, r10, 32-5, _PAGE_PRESENT
 	and	r11, r11, r10
@@ -353,6 +356,7 @@ DataStoreTLBMiss:
 	 * above.
 	 */
 	rlwimi	r11, r10, 0, _PAGE_GUARDED
+	rlwimi	r11, r10, 32 - 9, _PMD_PAGE_512K
 	mtspr	SPRN_MD_TWC, r11
 
 	/* Both _PAGE_ACCESSED and _PAGE_PRESENT has to be set.
@@ -584,7 +588,6 @@ FixupDAR:/* Entry point for dcbx workaround. */
 	mfspr	r11, SPRN_MD_TWC
 	lwz	r11, 0(r11)	/* Get the pte */
 	bt	28,200f		/* bit 28 = Large page (8M) */
-	bt	29,202f		/* bit 29 = Large page (8M or 512K) */
 	/* concat physical page address(r11) and page offset(r10) */
 	rlwimi	r11, r10, 0, 32 - PAGE_SHIFT, 31
 201:	lwz	r11,0(r11)
@@ -611,11 +614,6 @@ FixupDAR:/* Entry point for dcbx workaround. */
 	rlwimi	r11, r10, 0, 32 - PAGE_SHIFT_8M, 31
 	b	201b
 
-202:
-	/* concat physical page address(r11) and page offset(r10) */
-	rlwimi	r11, r10, 0, 32 - PAGE_SHIFT_512K, 31
-	b	201b
-
 144:	mfspr	r10, SPRN_DSISR
 	rlwinm	r10, r10,0,7,5	/* Clear store bit for buggy dcbst insn */
 	mtspr	SPRN_DSISR, r10