Subject: Re: [PATCH -V5 06/13] arch/powerpc: Increase the slice range to 64TB

From: Paul Mackerras <[email-masked]>
Date: Wed, 1 Aug 2012 15:16:17 +1000

On Mon, Jul 30, 2012 at 04:52:12PM +0530, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <[email-masked]>
> 
> This patch makes the high psizes mask as an unsigned char array
> so that we can have more than 16TB. Currently we support upto
> 64TB

Comments below...

> diff --git a/arch/powerpc/mm/slb_low.S b/arch/powerpc/mm/slb_low.S
> index b9ee79ce..c355af6 100644
> --- a/arch/powerpc/mm/slb_low.S
> +++ b/arch/powerpc/mm/slb_low.S
> @@ -108,17 +108,34 @@ END_MMU_FTR_SECTION_IFCLR(MMU_FTR_1T_SEGMENT)
>  	 * between 4k and 64k standard page size
>  	 */
>  #ifdef CONFIG_PPC_MM_SLICES
> +	/* r10 have esid */
>  	cmpldi	r10,16
> -
> -	/* Get the slice index * 4 in r11 and matching slice size mask in r9 */
> -	ld	r9,PACALOWSLICESPSIZE(r13)
> -	sldi	r11,r10,2
> +	/* below SLICE_LOW_TOP */
>  	blt	5f
> -	ld	r9,PACAHIGHSLICEPSIZE(r13)
> -	srdi	r11,r10,(SLICE_HIGH_SHIFT - SLICE_LOW_SHIFT - 2)
> -	andi.	r11,r11,0x3c
> -
> -5:	/* Extract the psize and multiply to get an array offset */
> +	/*
> +	 * Handle hpsizes,
> +	 * r9 is get_paca()->context.high_slices_psize[index], r11 is mask_index
> +	 * We use r10 here, later we restore it to esid.
> +	 * Can we use other register instead of r10 ?

Only r9, r10 and r11 are available here, and you're using them all.
Restoring r10 with one integer instruction is going to be quicker than
saving and restoring another register to/from memory.

> +	 */
> +	srdi    r10,r10,(SLICE_HIGH_SHIFT - SLICE_LOW_SHIFT) /* index */
> +	srdi	r11,r10,1			/* r11 is array index */
> +	addi	r9,r11,PACAHIGHSLICEPSIZE
> +	lbzx	r9,r9,r13			/* r9 is hpsizes[r11] */
> +	sldi    r11,r11,1
> +	subf	r11,r11,r10	/* mask_index = index - (array_index << 1) */
> +	srdi	r10,r3,28	/* restore r10 with esid */
> +	b	6f

How about (untested):

	srdi    r11,r10,(SLICE_HIGH_SHIFT - SLICE_LOW_SHIFT + 1) /* index */
	addi	r9,r11,PACAHIGHSLICEPSIZE
	lbzx	r9,r13,r9			/* r9 is hpsizes[r11] */
	/* r11 = (r10 >> 12) & 1, i.e. grab lowest bit of 1T ESID */
	rldicl	r11,r10,(64 - (SLICE_HIGH_SHIFT - SLICE_LOW_SHIFT)),63
	b	6f

Note that I swapped the RA and RB arguments for the lbzx.  Our recent
processors process indexed mode instructions more quickly if the value
in RB is small.

>  static struct slice_mask slice_mask_for_size(struct mm_struct *mm, int psize)
>  {
> +	unsigned char *hpsizes;
> +	int index, mask_index;
>  	struct slice_mask ret = { 0, 0 };
>  	unsigned long i;
> -	u64 psizes;
> +	u64 lpsizes;
>  
> -	psizes = mm->context.low_slices_psize;
> +	lpsizes = mm->context.low_slices_psize;
>  	for (i = 0; i < SLICE_NUM_LOW; i++)
> -		if (((psizes >> (i * 4)) & 0xf) == psize)
> +		if (((lpsizes >> (i * 4)) & 0xf) == psize)
>  			ret.low_slices |= 1u << i;
>  
> -	psizes = mm->context.high_slices_psize;
> -	for (i = 0; i < SLICE_NUM_HIGH; i++)
> -		if (((psizes >> (i * 4)) & 0xf) == psize)
> +	hpsizes = mm->context.high_slices_psize;
> +	for (i = 0; i < SLICE_NUM_HIGH; i++) {
> +		mask_index = i & 0x1;
> +		index = i >> 1;
> +		if (((hpsizes[index] >> (mask_index * 4)) & 0xf) == psize)
>  			ret.high_slices |= 1u << i;

This needs to be 1ul not 1u, since we are creating a 64-bit mask.

>  static void slice_convert(struct mm_struct *mm, struct slice_mask mask, int psize)
>  {
> +	int index, mask_index;
>  	/* Write the new slice psize bits */
> -	u64 lpsizes, hpsizes;
> +	unsigned char *hpsizes;
> +	u64 lpsizes;
>  	unsigned long i, flags;
>  
>  	slice_dbg("slice_convert(mm=%p, psize=%d)\n", mm, psize);
> @@ -201,14 +208,18 @@ static void slice_convert(struct mm_struct *mm, struct slice_mask mask, int psiz
>  			lpsizes = (lpsizes & ~(0xful << (i * 4))) |
>  				(((unsigned long)psize) << (i * 4));
>  
> +	/* Assign the value back */
> +	mm->context.low_slices_psize = lpsizes;
> +
>  	hpsizes = mm->context.high_slices_psize;
> -	for (i = 0; i < SLICE_NUM_HIGH; i++)
> +	for (i = 0; i < SLICE_NUM_HIGH; i++) {
> +		mask_index = i & 0x1;
> +		index = i >> 1;
>  		if (mask.high_slices & (1u << i))

Again, 1ul now.  Check all the other places where we manipulate a
slice mask to see if there are any other instances of 1u that need to
be changed.

Paul.