Subject: Re: [RFC PATCH -V2 08/21] powerpc: Decode the pte-lp-encoding bits correctly.

From: Paul Mackerras <[email-masked]>
Date: Fri, 22 Feb 2013 16:37:35 +1100

On Thu, Feb 21, 2013 at 10:17:15PM +0530, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <[email-masked]>
> 
> We look at both the segment base page size and actual page size and store
> the pte-lp-encodings in an array per base page size.
> 
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>

This needs more than 2 lines of patch description.  In fact what
you're doing is adding general mixed page-size segment (MPSS)
support.  Doing this should mean that you can also get rid of the
MMU_PAGE_64K_AP value from the list in asm/mmu.h.

>  struct mmu_psize_def
>  {
>  	unsigned int	shift;	/* number of bits */
> -	unsigned int	penc;	/* HPTE encoding */
> +	unsigned int	penc[MMU_PAGE_COUNT];	/* HPTE encoding */

I guess this is reasonable, though adding space for 14 page size
encodings seems a little bit over the top.  Also, you don't seem to
have any way to indicate which encodings are valid, since 0 is a valid
encoding.  Maybe you need to add a valid bit higher up to indicate
which page sizes are valid.

> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 71d0c90..d2c9932 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -1515,7 +1515,12 @@ static void kvmppc_add_seg_page_size(struct kvm_ppc_one_seg_page_size **sps,
>  	(*sps)->page_shift = def->shift;
>  	(*sps)->slb_enc = def->sllp;
>  	(*sps)->enc[0].page_shift = def->shift;
> -	(*sps)->enc[0].pte_enc = def->penc;
> +	/*
> +	 * FIXME!!
> +	 * This is returned to user space. Do we need to
> +	 * return details of MPSS here ?

Yes, we do, probably a separate entry for each valid base/actual page
size pair.

> +static inline int hpte_actual_psize(struct hash_pte *hptep, int psize)
> +{
> +	unsigned int mask;
> +	int i, penc, shift;
> +	/* Look at the 8 bit LP value */
> +	unsigned int lp = (hptep->r >> LP_SHIFT) & ((1 << (LP_BITS + 1)) - 1);

Why LP_BITS + 1 here?  You seem to be extracting and comparing 9 bits
rather than 8.  Why is that?

> @@ -395,12 +422,13 @@ static void hpte_decode(struct hash_pte *hpte, unsigned long slot,
>  			/* valid entries have a shift value */
>  			if (!mmu_psize_defs[size].shift)
>  				continue;
> -
> -			if (penc == mmu_psize_defs[size].penc)
> -				break;
> +			for (a_size = 0; a_size < MMU_PAGE_COUNT; a_size++)
> +				if (penc == mmu_psize_defs[size].penc[a_size])
> +					goto out;

I think this will get false matches due to unused/invalid entries
in mmu_psize_defs[size].penc[] containing 0.

Paul.