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

Le 18/09/2021 à 05:26, Michael Ellerman a écrit :

Christophe Leroy [off-list ref] writes:

quoted

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 ...

quoted

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