Re: [PATCH v2] powerpc/32: Don't use a struct based type for pte_t

[Christophe Leroy]

Le 18/09/2021 à 05:26, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@xxxxxxxxxx> 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?


I think Kconfig doesn't see __CHECKER__, otherwise it would mean that a 
build may get different whether you build with C=1/2 or not.

__CHECKER__ is a define added by sparse when doing the CHECK on a per 
object basis.

What I can do is to add:

#if defined(__CHECKER__) || !defined(CONFIG_PPC32)
#define STRICT_MM_TYPECHECKS
#endif

Christophe