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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2] powerpc/32: Don't use a struct based type for pte_t
In-Reply-To: <c904599f33aaf6bb7ee2836a9ff8368509e0d78d.1631887042.git.christophe.leroy@csgroup.eu>
References: <c904599f33aaf6bb7ee2836a9ff8368509e0d78d.1631887042.git.christophe.leroy@csgroup.eu>
Date:   Sat, 18 Sep 2021 13:26:57 +1000
Message-ID: <87tuiiimwu.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
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Precedence: bulk

Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> 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.
>
> Commit 670eea924198 ("powerpc/mm: Always use STRICT_MM_TYPECHECKS")
> made the struct based definition the only one, considering that the
> generated code was similar in both cases.
>
> That's right on ppc64 because the ABI is such that the content of a
> struct having a single simple type element is passed as register,
> but on ppc32 such a structure is passed via the stack like any
> structure.
>
> Simple test function:
>
> 	pte_t test(pte_t pte)
> 	{
> 		return pte;
> 	}
>
> Before this patch we get
>
> 	c00108ec <test>:
> 	c00108ec:	81 24 00 00 	lwz     r9,0(r4)
> 	c00108f0:	91 23 00 00 	stw     r9,0(r3)
> 	c00108f4:	4e 80 00 20 	blr
>
> So, for PPC32, restore the simple type behaviour we got before
> commit 670eea924198, but instead of adding a config option to
> activate type check, do it when __CHECKER__ is set so that type
> checking is performed by 'sparse' and provides feedback like:
>
> 	arch/powerpc/mm/pgtable.c:466:16: warning: incorrect type in return expression (different base types)
> 	arch/powerpc/mm/pgtable.c:466:16:    expected unsigned long
> 	arch/powerpc/mm/pgtable.c:466:16:    got struct pte_t [usertype] x

OK that's a good trade off.

One question below ...

> 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)

It would be nicer if this logic was in Kconfig.

eg. restore config STRICT_MM_TYPECHECKS but make it always enabled for
64-bit, and depend on CHECKER for 32-bit.

The only thing is I'm not sure if we can test __CHECKER__ in Kconfig?

cheers