[02/11] powerpc/8xx: remove tests on PGDIR entry validity

Kernel MMU handling code handles validity of entries via _PMD_PRESENT which
corresponds to V bit in MD_TWC and MI_TWC. When the V bit is not set, MPC8xx
triggers TLBError exception. So we don't have to check that and branch ourself
to TLBError. We can set TLB entries with non present entries, remove all those
tests and let the 8xx handle it. This reduce the number of cycle when the
entries are valid which is the case most of the time, and doesn't significantly
increase the time for handling invalid entries.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>

---
 arch/powerpc/kernel/head_8xx.S | 41 ++++++++---------------------------------
 1 file changed, 8 insertions(+), 33 deletions(-)

On Tue, 2014-12-16 at 16:03 +0100, Christophe Leroy wrote:
> Kernel MMU handling code handles validity of entries via _PMD_PRESENT which
> corresponds to V bit in MD_TWC and MI_TWC. When the V bit is not set, MPC8xx
> triggers TLBError exception. So we don't have to check that and branch ourself
> to TLBError. We can set TLB entries with non present entries, remove all those
> tests and let the 8xx handle it. This reduce the number of cycle when the
> entries are valid which is the case most of the time, and doesn't significantly
> increase the time for handling invalid entries.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>

Nice :)
Acked-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>

> 
> ---
>  arch/powerpc/kernel/head_8xx.S | 41 ++++++++---------------------------------
>  1 file changed, 8 insertions(+), 33 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
> index 79b8a23..2c329f1 100644
> --- a/arch/powerpc/kernel/head_8xx.S
> +++ b/arch/powerpc/kernel/head_8xx.S
> @@ -329,12 +329,9 @@ InstructionTLBMiss:
>         /* Extract level 1 index */
>         rlwinm  r10, r10, 32 - ((PAGE_SHIFT - 2) << 1), (PAGE_SHIFT - 2) << 1, 29
>         lwzx    r11, r10, r11/* Get the level 1 entry */
> -       rlwinm. r10, r11,0,0,19/* Extract page descriptor page address */
> -       beq     2f      /* If zero, don't try to find a pte */
> +       rlwinm  r10, r11,0,0,19/* Extract page descriptor page address */
>  
> -       /* We have a pte table, so load the MI_TWC with the attributes
> -               * for this "segment."
> -               */
> +       /* Load the MI_TWC with the attributes for this "segment." */
>         MTSPR_CPU6(SPRN_MI_TWC, r11, r3)/* Set segment attributes */
>         mfspr   r11, SPRN_SRR0/* Get effective address of fault */
>         /* Extract level 2 index */
> @@ -342,13 +339,11 @@ InstructionTLBMiss:
>         lwzx    r10, r10, r11/* Get the pte */
>  
>  #ifdef CONFIG_SWAP
> -       andi.   r11, r10, _PAGE_ACCESSED | _PAGE_PRESENT
> -       cmpwi   cr0, r11, _PAGE_ACCESSED | _PAGE_PRESENT
> -       li      r11, RPN_PATTERN
> -       bne-    cr0, 2f
> -#else
> -       li      r11, RPN_PATTERN
> +       rlwinm  r11, r10, 32-5, _PAGE_PRESENT
> +       and     r11, r11, r10
> +       rlwimi  r10, r11, 0, _PAGE_PRESENT
>  #endif
> +       li      r11, RPN_PATTERN
>         /* The Linux PTE won't go exactly into the MMU TLB.
>                 * Software indicator bits 21 and 28 must be clear.
>                 * Software indicator bits 24, 25, 26, and 27 must be
> @@ -366,21 +361,6 @@ InstructionTLBMiss:
>         mfspr   r10, SPRN_SPRG_SCRATCH2
>         EXCEPTION_EPILOG_0
>         rfi
> -2:
> -       mfspr   r10, SPRN_SRR1
> -       /* clear all error bits as TLB Miss
> -               * sets a few unconditionally
> -       */
> -       rlwinm  r10, r10, 0, 0xffff
> -       mtspr   SPRN_SRR1, r10
> -
> -       /* Restore registers */
> -#ifdef CONFIG_8xx_CPU6
> -       mfspr   r3, SPRN_DAR
> -       mtspr   SPRN_DAR, r11/* Tag DAR */
> -#endif
> -       mfspr   r10, SPRN_SPRG_SCRATCH2
> -       b       InstructionTLBError1
>  
>         . = 0x1200
>  DataStoreTLBMiss:
> @@ -403,8 +383,6 @@ DataStoreTLBMiss:
>         /* Extract level 1 index */
>         rlwinm  r10, r10, 32 - ((PAGE_SHIFT - 2) << 1), (PAGE_SHIFT - 2) << 1, 29
>         lwzx    r11, r10, r11/* Get the level 1 entry */
> -       rlwinm. r10, r11,0,0,19/* Extract page descriptor page address */
> -       beq     2f      /* If zero, don't try to find a pte */
>  
>         /* We have a pte table, so load fetch the pte from the table.
>                 */
> @@ -450,7 +428,7 @@ DataStoreTLBMiss:
>                 * set.  All other Linux PTE bits control the behavior
>                 * of the MMU.
>                 */
> -2:     li      r11, RPN_PATTERN
> +       li      r11, RPN_PATTERN
>         rlwimi  r10, r11, 0, 24, 28/* Set 24-27, clear 28 */
>         MTSPR_CPU6(SPRN_MD_RPN, r10, r3)/* Update TLB entry */
>  
> @@ -469,10 +447,7 @@ DataStoreTLBMiss:
>   */
>         . = 0x1300
>  InstructionTLBError:
> -       EXCEPTION_PROLOG_0
> -InstructionTLBError1:
> -       EXCEPTION_PROLOG_1
> -       EXCEPTION_PROLOG_2
> +       EXCEPTION_PROLOG
>         mr      r4,r12
>         mr      r5,r9
>         andis.  r10,r5,0x4000