Android and compatibility with deprecated armv7 instructions

[Catalin Marinas]

On Fri, Jul 04, 2014 at 08:08:05AM +0100, Ard Biesheuvel wrote:
> On 4 July 2014 00:47, Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
> > On Thu, Jul 03, 2014 at 11:16:16PM +0100, Måns Rullgård wrote:
> >> Will Deacon <will.deacon at arm.com> writes:
> >>
> >> > On Thu, Jul 03, 2014 at 06:05:58PM +0100, Russell King - ARM Linux wrote:
> >> >> On Thu, Jul 03, 2014 at 05:22:30PM +0100, Grant Likely wrote:
> >> >> > So, no. I completely reject any notion that breaking existing apps is
> >> >> > okay. If we're going to say that v8 still supports 32-bit apps, then
> >> >> > it has to be all of v7, not just the 'good' bits. Nor do I think
> >> >> > saying "it's just a bunch of games" justifies anything. We're kernel
> >> >> > engineers. Applications are applications and we don't break userspace.
> >> >> > Period.
> >> >>
> >> >> +1 on all points above.  I'd go further - if we're going to say that v8
> >> >> still supports 32-bit apps, that covers at least v6 *as well*.
> >> >
> >> > We've never pretended to support anything other than ARMv8 in the compat
> >> > layer. uname even reports this in the machine name.
> >> >
> >> > If people are suddenly so concerned about *full* compatibility with an ARMv7
> >> > kernel, that needs a lot more than just SWP emulation:
> >> >
> >> >   - Alignment fixups for ldm/stm
> >>
> >> No ARM variant ever supported unaligned ldm/stm.
> >
> > Quite right but that's not the point being discussed.  Please note that
> > the sentence says "with an ARMv7 *kernel*" - we are not talking about
> > the architecture there.
> >
> > So, what's more to the point is that on 32-bit ARM userspace under Linux,
> > we _have_ supported it since early 2000 up to present.  It's not currently
> > supported on 64-bit ARM running Linux, even when running a 32-bit binary
> > in userspace.
> >
> > Ergo, it's a user visible ABI change, one which we don't know whether it
> > matters.  In all probability, it doesn't because (hopefully) no one ever
> > does unaligned LDMs/STMs - I think it would require hand-crafted assembly,
> > at which point you're talking about optimising something, and you'd be
> > silly to do it as it would invoke the alignment fault handling which
> > would be slow.
> >
> 
> Well, if something like this
> 
> struct bar {
>     long l[4];
> };
> 
> void foo(struct bar *dst, struct bar const *src)
> {
>     *dst = *src;
> }
> 
> produces this:
> 
> foo:
> @ args = 0, pretend = 0, frame = 0
> @ frame_needed = 0, uses_anonymous_args = 0
> @ link register save eliminated.
>     mov ip, r0
>     ldmia r1, {r0, r1, r2, r3}
>     stmia ip, {r0, r1, r2, r3}
>     bx lr
> 
> won't it take just a single cast from some unaligned char* to struct
> bar* to trigger this?

Is this even allowed by the C ABI? The compiler generates the LDMs
because function foo() gets a struct pointer which is guaranteed to be
aligned.

-- 
Catalin