Re: [linuxsh-dev] ?? Using tWrite-Through bit in PTE with 2.4.24 ??

[Paul M. <pau...@no...>]

On Fri, Feb 04, 2005 at 07:28:47PM +0000, Stuart MENEFY wrote:
> On Fri, 4 Feb 2005 17:36:11 +0200 Paul Mundt <pau...@no...> wrote:
> > Eek, no. This is the fundamental basis for PAGE_SHARED, which we need f=
or
> > page protection (see for instance the mmap protection_map).
>=20
> Are you sure? Back in the dim and distant past (2.3.? - 2.4.5) this was
> used to implement cache flushing of shared pages to overcome D-cache
> aliasing problems. However in recent 2.4 and 2.6 kernels I can't find
> anything which actually uses it. Yes, it is set, but I can't find anything
> which ever tests it.
>=20
Looking into early 2.4, there was a software _PAGE_SHARED bit, it seems that
this was being used for pte_shared() (with appropriate dcache purging in
update_mmu_cache()), which seemed fairly short lived.

_PAGE_U0_SHARED was introduced at 2.4.1-pre9 time by NIIBE-san:

2001-01-23  NIIBE Yutaka  <gn...@m1...>

	[ ... ]

        * include/asm-sh/pgtable.h (_PAGE_SHARED): Always _PAGE_U0_SHARED.
        (was conditionally _PAGE_HW_SHARED on SH-3).  With _PAGE_HW_SHARED,
        all processes share the page, while proper semantics is "some
        processes share the page".
        (flush_cache_sigtramp): New function.

        * Updated to 2.4.1-pre9.

	[ ... ]

since then this is being used for _PAGE_SHARED, but looking closer, I likew=
ise
don't see anything explicitly testing for it.

pte_shared() was later removed, but the _PAGE_SHARED stuff remained:

2001-06-30  NIIBE Yutaka  <gn...@m1...>

	[ ... ]

        * arch/sh/mm/fault.c (update_mmu_cache): Removed pte_shared
        handling.
        (__flush_tlb_phys): Removed.
        * include/asm-sh/pgtable.h (pte_shared): Removed.

Given that, this is probably a left over artifact of pte_shared() and with
that gone, we can probably get away with reallocating this bit for _PAGE_FI=
LE
and freeing up the WT bit (though this will probably want some testing to m=
ake
sure that things like MAP_SHARED mmaps aren't breaking all over themselves).

Perhaps something like this (completely untested)..

--- orig/include/asm-sh/pgtable.h
+++ mod/include/asm-sh/pgtable.h
@@ -64,10 +64,8 @@
 #define _PAGE_SZ1	0x080  /* SZ1-bit : Size of page (on SH-4) */
 #define _PAGE_PRESENT	0x100  /* V-bit   : page is valid */
 #define _PAGE_PROTNONE	0x200  /* software: if not present  */
-#define _PAGE_ACCESSED 	0x400  /* software: page referenced */
-#define _PAGE_U0_SHARED 0x800  /* software: page is shared in user space */
-
-#define	_PAGE_FILE	_PAGE_WT  /* software: pagecache or swap? */
+#define _PAGE_ACCESSED	0x400  /* software: page referenced */
+#define _PAGE_FILE	0x800  /* software: pagecache or swap? */
=20
 /* software: moves to PTEA.TC (Timing Control) */
 #define _PAGE_PCC_AREA5	0x00000000	/* use BSC registers for area5 */
@@ -83,11 +81,8 @@
 #define _PAGE_PCC_ATR16	0x60000001	/* Attribute Memory space, 6 bit bus */
=20
=20
-/* Mask which drop software flags
- * We also drop WT bit since it is used for _PAGE_FILE
- * bit in this implementation.
- */
-#define _PAGE_CLEAR_FLAGS	(_PAGE_WT | _PAGE_PROTNONE | _PAGE_ACCESSED | _P=
AGE_U0_SHARED)
+/* Mask which drop software flags */
+#define _PAGE_CLEAR_FLAGS	(_PAGE_PROTNONE | _PAGE_ACCESSED | _PAGE_FILE)
=20
 #if defined(CONFIG_CPU_SH3)
 /*
@@ -108,15 +103,13 @@
 #define _PAGE_SZHUGE	(_PAGE_SZ0 | _PAGE_SZ1)
 #endif
=20
-#define _PAGE_SHARED	_PAGE_U0_SHARED
-
 #define _PAGE_TABLE	(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER | _PAGE_ACCESSE=
D | _PAGE_DIRTY)
 #define _KERNPG_TABLE	(_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED | _PAGE_D=
IRTY)
-#define _PAGE_CHG_MASK	(PTE_MASK | _PAGE_ACCESSED | _PAGE_CACHABLE | _PAGE=
_DIRTY | _PAGE_SHARED)
+#define _PAGE_CHG_MASK	(PTE_MASK | _PAGE_ACCESSED | _PAGE_CACHABLE | _PAGE=
_DIRTY)
=20
 #ifdef CONFIG_MMU
 #define PAGE_NONE	__pgprot(_PAGE_PROTNONE | _PAGE_CACHABLE |_PAGE_ACCESSED=
 | _PAGE_FLAGS_HARD)
-#define PAGE_SHARED	__pgprot(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER | _PAGE=
_CACHABLE |_PAGE_ACCESSED | _PAGE_SHARED | _PAGE_FLAGS_HARD)
+#define PAGE_SHARED	__pgprot(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER | _PAGE=
_CACHABLE |_PAGE_ACCESSED | _PAGE_FLAGS_HARD)
 #define PAGE_COPY	__pgprot(_PAGE_PRESENT | _PAGE_USER | _PAGE_CACHABLE | _=
PAGE_ACCESSED | _PAGE_FLAGS_HARD)
 #define PAGE_READONLY	__pgprot(_PAGE_PRESENT | _PAGE_USER | _PAGE_CACHABLE=
 | _PAGE_ACCESSED | _PAGE_FLAGS_HARD)
 #define PAGE_KERNEL	__pgprot(_PAGE_PRESENT | _PAGE_RW | _PAGE_CACHABLE | _=
PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_HW_SHARED | _PAGE_FLAGS_HARD)
@@ -209,6 +202,7 @@
 }
=20
 #define pgprot_writecombine(prot) __pgprot(pgprot_val(prot) & ~_PAGE_CACHA=
BLE)
+#define pgprot_writethrough(prot) __pgprot(pgprot_val(prot) | _PAGE_WT)
=20
 /*
  * Conversion functions: convert a page and protection to a page entry,