[RFA:] Caveat for ARM in gcc-4.7/changes.html: unaligned accesses

[Hans-Peter Nilsson]

> From: Michael Hope <michael.hope@linaro.org>
> Date: Wed, 13 Jun 2012 00:43:47 +0200

> On 13 June 2012 02:32, Hans-Peter Nilsson <hans-peter.nilsson@axis.com> wrote:
> >> From: Hans-Peter Nilsson <hp@axis.com>
> >> Date: Mon, 11 Jun 2012 00:59:57 +0200
> >> > > user-space code". Â Maybe the kernel too, I can't say; there was
> >> > > IIRC no sign of it.
> > But (at least) after removing some local changed defaults,
> > there's at boot-time a lot of:
> >
> > [ Â  Â 0.950000] Unhandled fault: alignment exception (0x801) at 0xc821ddee
> 
> That's a kernel address.

Yes I know, there's no mystery there.  My point was that
misalignment traps for this configuration happen at boot time
too, not just from userspace, hence correcting my earlier
statement.

> For reference, the message comes from
> arch/arm/mm/alignment.c:alignment_init() from the default trap
> handler.

No news there.

> The lines just before this disable the unaligned trap for
> usermode:
> 
> 	if (cpu_is_v6_unaligned()) {
> 		cr_alignment &= ~CR_A;
> 		cr_no_alignment &= ~CR_A;
> 		set_cr(cr_alignment);
> 		ai_usermode = safe_usermode(ai_usermode, false);
> 	}
> 
> Support was added by Russell King in 2008-12 and updated by Dave
> Martin on 2011-07.

I see it, for example in stock 2.6.35.9.

> Out of interest, does your CPU report support for unaligned access via
> CP15 CR1?  It's bit 22 and shows during boot.  My board shows:
> 
> CPU: ARMv7 Processor [411fc092] revision 2 (ARMv7), cr=50c5387d

The log that my colleague sent me contains:
[    0.000000] CPU: ARMv6-compatible processor [4117b365] revision 5 (ARMv6TEJ), cr=00c5387f

Bit 22 all set...

> This compiles into a five byte unaligned memcpy:

Yes, that was the point. :)

> Linaro's stock configuration is -march=armv7-a -mtune=cortex-a9
> -mthumb.  Ubuntu is the same.  I can't reproduce the fault on a
> PandaBoard with omapzoom 2.6.35, Ubuntu 3.2.14, Ubuntu Precise 4.6.3
> GCC, or plain gcc-4.7.1-RC-20120606.

Looking at arch/arm/mm/alignment.c:alignment_init(), I see we
have local patches always forcing a warning.  Bah.  Is your
point that having an OS kernel that traps for unaligned accesses
for ARMv6 rare enough that there's no reason whatsoever for a
release note that gcc-4.7 now emits such accesses "by itself"?

Even though this was caused by non-stock Linux, I think there's
reason enough for a caveat entry in gcc-4.7/changes.html (just
as worded; the caveat wasn't directed just at Linux, mind you),
but I retract my suggestion to change back the default.

Maybe we should add a line to the suggested entry mentioning the
first stock Linux version that automatically adjusts this, so
stock Linux-users can be calmed. ;-P  Do you agree?

Something like "Linux/ARM in official releases has automatically
and unconditionally supported unaligned accesses as emitted by
GCC due to this option being active since Linux version 2.X.Y".
I guess X is 6.  What is Y?

brgds, H-P