Re: [PATCH 1/8] powerpc/rtas: ensure 8-byte alignment for struct rtas_args

[Andrew Donnellan <ajd@linux.ibm.com>]

On Mon, 2023-03-06 at 15:33 -0600, Nathan Lynch via B4 Relay wrote:
> > From: Nathan Lynch <nathanl@linux.ibm.com>
> > 
> > CHRP and PAPR agree: "In order to make an RTAS call, the operating
> > system must construct an argument call buffer aligned on an eight
> > byte
> > boundary in physically contiguous real memory [...]." (7.2.7
> > Calling
> > Mechanism and Conventions).
> > 
> > struct rtas_args is the type used for this argument call buffer.
> > The
> > unarchitected 'rets' member happens to produce 8-byte alignment for
> > the struct on 64-bit targets in practice. But without an alignment
> > directive the structure will have only 4-byte alignment on 32-bit
> > targets:
> > 
> >   $ nm b/{before,after}/chrp32/vmlinux | grep rtas_args
> >   c096881c b rtas_args
> >   c0968820 b rtas_args
> > 
> > Add an alignment directive to the struct rtas_args declaration so
> > all
> > instances have the alignment required by the specs. rtas-types.h no
> > longer refers to any spinlock types, so drop the spinlock_types.h
> > inclusion while we're here.
> > 
> > Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>

Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>

> > ---
> >  arch/powerpc/include/asm/rtas-types.h | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/rtas-types.h
> > b/arch/powerpc/include/asm/rtas-types.h
> > index f2ad4a96cbc5..861145c8a021 100644
> > --- a/arch/powerpc/include/asm/rtas-types.h
> > +++ b/arch/powerpc/include/asm/rtas-types.h
> > @@ -2,7 +2,8 @@
> >  #ifndef _ASM_POWERPC_RTAS_TYPES_H
> >  #define _ASM_POWERPC_RTAS_TYPES_H
> >  
> > -#include <linux/spinlock_types.h>
> > +#include <linux/compiler_attributes.h>
> > +#include <linux/sizes.h>
> >  
> >  typedef __be32 rtas_arg_t;
> >  
> > @@ -12,7 +13,7 @@ struct rtas_args {
> >         __be32 nret;
> >         rtas_arg_t args[16];
> >         rtas_arg_t *rets;     /* Pointer to return values in
> > args[].
> > */
> > -};
> > +} __aligned(SZ_8);

Nowhere else in the kernel uses __aligned(SZ_8) over just __aligned(8),
which I suppose would also save an include, but I don't care either
way.

> >  
> >  struct rtas_t {
> >         unsigned long entry;            /* physical address pointer
> > */
> > 

-- 
Andrew Donnellan    OzLabs, ADL Canberra
ajd@linux.ibm.com   IBM Australia Limited