Subject: Re: [PATCH v2] powerpc: 64K page support for kexec

From: [email-masked] (Olof Johansson)
Date: Thu, 26 Apr 2007 02:15:32 -0500

Hi,
                                                                                                                                             
Two comments below, besides that it looks good!
                                                                                                                                             
On Wed, Apr 25, 2007 at 04:35:39PM -0300, Luke Browning wrote:

> +#define LP_SHIFT	12
> +#define LP_BITS		8
> +#define LP_MASK(i)	((0xFF >> (i)) << LP_SHIFT)
> +
> +static void hpte_decode(hpte_t *hpte, unsigned long slot, 
> +			int *psize, unsigned long *va)
> +{
> +	unsigned long hpte_r = hpte->r;
> +	unsigned long hpte_v = hpte->v;
> +	unsigned long avpn;
> +	int i, size, shift, penc, avpnm_bits;
> +		
> +	if (!(hpte_v & HPTE_V_LARGE))
> +		size = MMU_PAGE_4K;
> +	else {
> +		for (i = 0; i < LP_BITS; i++) {
> +			if ((hpte_r & LP_MASK(i+1)) == LP_MASK(i+1))
> +				break;
> +		}
> +		penc = LP_MASK(i+1) >> LP_SHIFT;
> +		for (size = 0; size < MMU_PAGE_COUNT - 1; size++) {

This will never consider the last page size. Either <= MMU_PAGE_COUNT-1
or < MMU_PAGE_COUNT. The latter would be more natural.

> -		pteg = slot / HPTES_PER_GROUP;
> -		if (hpte_v & HPTE_V_SECONDARY)
> -			pteg = ~pteg;
> +			/* 4K pages are not represented by LP */
> +			if (size == MMU_PAGE_4K)
> +				continue;
> +
> +			/* valid entries have a shift value */
> +			if (!mmu_psize_defs[size].shift)
> +				continue;
>  
> -		vpi = ((va >> 28) ^ pteg) & htab_hash_mask;
> +			if (penc == mmu_psize_defs[size].penc)
> +				break;
> +		}
> +	}
>  
> -		va |= vpi << PAGE_SHIFT;
> +	/*
> +	 * FIXME, the code below works for 16M, 64K, and 4K pages as these
> +	 * fall under the p<=23 rules for calculating the virtual address.
> +	 * In the case of 16M pages, an extra bit is stolen from the AVPN
> +	 * field to achieve the requisite 24 bits. 
> +	 *
> +	 * 16G pages are not supported by the code below.
> +	 */
> +	BUG_ON(hpte_v & 0x4000000000000000UL);		/* 1T segment */
> +	BUG_ON(size == MMU_PAGE_16G);
> +	BUG_ON(size == MMU_PAGE_64K_AP);

Milton didn't like BUG_ON() here, and I think I agree after hearing his
motivations. I know I was the one suggesting them, but they're better
to keep out at the moment.

[...]

Thanks,

-Olof