linux-kernel - RE: [PATCH v2] powerpc/32: Don't use a struct based type for pte

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Message-ID: <505920070e5f4bf8ad7ccaa12f346469@AcuMS.aculab.com>
Date:   Fri, 17 Sep 2021 14:32:22 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christophe Leroy' <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        "Michael Ellerman" <mpe@...erman.id.au>
CC:     "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] powerpc/32: Don't use a struct based type for pte_t

From: Christophe Leroy
> Sent: 17 September 2021 14:58
> 
> Long time ago we had a config item called STRICT_MM_TYPECHECKS
> to build the kernel with pte_t defined as a structure in order
> to perform additional build checks or build it with pte_t
> defined as a simple type in order to get simpler generated code.
> 
...
> diff --git a/arch/powerpc/include/asm/pgtable-types.h b/arch/powerpc/include/asm/pgtable-types.h
> index d11b4c61d686..c60199fc6fa6 100644
> --- a/arch/powerpc/include/asm/pgtable-types.h
> +++ b/arch/powerpc/include/asm/pgtable-types.h
> @@ -5,14 +5,26 @@
>  /* PTE level */
>  #if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
>  typedef struct { pte_basic_t pte, pte1, pte2, pte3; } pte_t;
> -#else
> +#elif defined(__CHECKER__) || !defined(CONFIG_PPC32)
>  typedef struct { pte_basic_t pte; } pte_t;
> +#else
> +typedef pte_basic_t pte_t;
>  #endif
> +
> +#if defined(__CHECKER__) || !defined(CONFIG_PPC32) || \
> +    (defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES))
>  #define __pte(x)	((pte_t) { (x) })
>  static inline pte_basic_t pte_val(pte_t x)
>  {
>  	return x.pte;
>  }
> +#else
> +#define __pte(x)	((pte_t)(x))
> +static inline pte_basic_t pte_val(pte_t x)
> +{
> +	return x;
> +}
> +#endif

Would it be better to define:
static inline pte_basic_*pte_basic(pte_t *x)
{
#if xxx
	return x;
#else
	return &x->pte;
#endif
}

Then pte_val(x) is always *pt_basic(x)
and the casts like:

> -	pte_basic_t *entry = &ptep->pte;
> +	pte_basic_t *entry = (pte_basic_t *)ptep;

can go away.

	David

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