Subject: Re: [PATCH 2/6] 8xx: get rid of _PAGE_HWWRITE dependency in MMU.

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 08 Oct 2009 08:14:52 +1100

On Wed, 2009-10-07 at 22:46 +0200, Joakim Tjernlund wrote:

> +	andi.	r11, r10, _PAGE_USER | _PAGE_ACCESSED
> +	cmpwi	cr0, r11, _PAGE_USER | _PAGE_ACCESSED
> +	bne-	cr0, 2f

Did you mean _PAGE_PRESENT | _PAGE_ACCESSED ?

> +2:
> +	mfspr	r11, SRR1
> +	rlwinm	r11, r11, 0, 5, 3 /* clear guarded */
> +	mtspr	SRR1, r11

What is the above for ?

> +	mfspr	r10, SPRN_M_TW	/* Restore registers */
> +	lwz	r11, 0(r0)
> +	mtcr	r11
> +	lwz	r11, 4(r0)
> +#ifdef CONFIG_8xx_CPU6
> +	lwz	r3, 8(r0)
> +#endif
> +	b	InstructionAccess
>  

 .../...

> +	andi.	r11, r10, _PAGE_ACCESSED
> +	bne+	cr0, 5f	/* branch if access allowed */
> +	rlwinm	r10, r10, 0, 21, 19 /* Clear _PAGE_USER */
> +	ori	r10, r10, _PAGE_RW  /* Set RW bit for xor below to clear it */
> +5:	xori	r10, r10, _PAGE_RW  /* invert RW bit */

Why are you clearing _PAGE_USER ? One needs to differenciate usr
from kernel pages or user will be access to write to kernel...

Why don't you do the trick I proposed in my email with loading a
constant that contains all the bit combinations and shifting it
by the amount defined by _PAGE_RW and _PAGE_USER used as a two
bits index ?
 
>  	/* The Linux PTE won't go exactly into the MMU TLB.
> -	 * Software indicator bits 21, 22 and 28 must be clear.
> +	 * Software indicator bits 22 and 28 must be clear.
>  	 * Software indicator bits 24, 25, 26, and 27 must be
>  	 * set.  All other Linux PTE bits control the behavior
>  	 * of the MMU.
> @@ -472,8 +470,8 @@ DataTLBError:
>  	/* First, make sure this was a store operation.
>  	*/
>  	mfspr	r10, SPRN_DSISR
> -	andis.	r11, r10, 0x4800 /* no translation, no permission. */
> -	bne	2f	/* branch if either is set */
> +	andis.	r11, r10, 0x4000 /* no translation */
> +	bne	2f	/* branch if set */
>  
>  	/* The EA of a data TLB miss is automatically stored in the MD_EPN
>  	 * register.  The EA of a data TLB error is automatically stored in
> @@ -522,26 +520,20 @@ DataTLBError:
>  	mfspr	r11, SPRN_MD_TWC		/* ....and get the pte address */
>  	lwz	r10, 0(r11)		/* Get the pte */
>  
> -	andi.	r11, r10, _PAGE_RW	/* Is it writeable? */
> -	beq	2f			/* Bail out if not */
> -
> -	/* Update 'changed', among others.
> -	*/
> -#ifdef CONFIG_SWAP
> +	mfspr	r11, DSISR
> +	andis.	r11, r11, 0x0200	/* store */
> +	beq	5f
> +	andi.	r11, r10, _PAGE_RW	/* writeable? */
> +	beq	2f /* nope */
>  	ori	r10, r10, _PAGE_DIRTY|_PAGE_HWWRITE
> -	/* do not set the _PAGE_ACCESSED bit of a non-present page */
> -	andi.	r11, r10, _PAGE_PRESENT
> -	beq	4f
> -	ori	r10, r10, _PAGE_ACCESSED
> -4:
> -#else
> -	ori	r10, r10, _PAGE_DIRTY|_PAGE_ACCESSED|_PAGE_HWWRITE
> -#endif
> -	mfspr	r11, SPRN_MD_TWC		/* Get pte address again */
> +5:	ori	r10, r10, _PAGE_ACCESSED
> +	mfspr	r11, MD_TWC		/* Get pte address again */
>  	stw	r10, 0(r11)		/* and update pte in table */
>  
> +	xori	r10, r10, _PAGE_RW	/* RW bit is inverted */
> +
>  	/* The Linux PTE won't go exactly into the MMU TLB.
> -	 * Software indicator bits 21, 22 and 28 must be clear.
> +	 * Software indicator bits 22 and 28 must be clear.
>  	 * Software indicator bits 24, 25, 26, and 27 must be
>  	 * set.  All other Linux PTE bits control the behavior
>  	 * of the MMU.

I still don't find how any of the above is useful... why not just go
straight to C code ?

Cheers,
Ben.