DWord alignment on ARMv7

[arnd@arndb.de (Arnd Bergmann)]

On Friday 04 March 2016 12:44:23 Ard Biesheuvel wrote:
> On 4 March 2016 at 12:38, Arnd Bergmann <arnd@arndb.de> wrote:
> > On Friday 04 March 2016 12:14:24 Ard Biesheuvel wrote:
> >> On 4 March 2016 at 12:02, Russell King - ARM Linux
> > Here is a patch I've come up with independently. I have verified
> > that it removes all ldrd/strd from the btrfs unaligned data
> > handling.
> >
> > The open question about it is whether we'd rather play safe and
> > let the compiler handle unaligned accesses itself, removing the
> > theoretical risk of the compiler optimizing
> >
> >         void *p;
> >         u64 v = get_unaligned((u32)p) + (get_unaligned((u32)(p + 4)) << 32);
> >
> > into an ldrd. I think the linux/unaligned/access_ok.h implementation
> > would allow that.
> >
> 
> I would assume that the compiler engineers are aware of the alignment
> requirement of ldrd/strd, and don't promote adjacent accesses like
> that if the pointer may not be 64-bit aligned.

Ah, I thought it only required 32-bit alignment like ldm/stm, but it
seems that it won't do that. However, an implementation like

unsigned long long get_unaligned_u64(void *p)
{
        unsigned long long upper, lower;
        lower = *(unsigned long*)p;
        upper = *(unsigned long*)(p+4);

        return lower | (upper << 32);
}

does get compiled into

00000000 <f>:
   0:   e8900003        ldm     r0, {r0, r1}
   4:   e12fff1e        bx      lr

which is still wrong, so I assume there is some danger of that remaining
with both of our patches, as the compiler might  decide to merge
a series of unaligned 32-bit loads into an ldm, as long as our implementation
incorrectly tells the compiler that the data is 32-bit aligned.

> > + * This is the most generic implementation of unaligned accesses
> > + * and should work almost anywhere.
> > + */
> > +#include <asm/byteorder.h>
> > +#include <linux/kernel.h>
> > +#include <asm/byteorder.h>
> 
> Any particular reason to include this twice?

No, just a mistake when merging the access_ok.h into this file.

	Arnd