Re-opening the bug to discuss some new issues.
Both of these problems are effectively present in the previous Ubuntu
releases on imx51, as well as lucid.
1) CONFIG_NEON=n doesn't actually disable the use of NEON instructions
(I misinderstood things here). It should be straightforward to improve
the situation here: for platforms with known problems, or when
CONFIG_NEON=n, we can set the ASEDIS bit in the CP15 Coprocessor Access
Control Register on boot. This disables the part of the NEON
instruction space containing the instructions which can cause unaligned
accesses to occur.
2) It appears that all unaligned accesses (not just NEON) can cause
problems. This may apply to cached memory, but has not definitely been
observed. However, the problem is observable for uncached memory. This
means that memory-mapped drivers such as the framebuffer or media
accelerators can provide userspace processes with a way to hit the bug
using ordinary unaligned loads and stores.
The problem is that the kernel does not trap ordinary loads and stores
on ARMv6 and above, because these processors support the unaligned
access natively.
arch/arm/mm/alignment.c:
static int __init alignment_init(void)
{
...
if (cpu_architecture() >= CPU_ARCH_ARMv6 && (cr_alignment & CR_U)) {
cr_alignment &= ~CR_A;
cr_no_alignment &= ~CR_A;
set_cr(cr_alignment);
ai_usermode = UM_FIXUP;
}
In principle we could support the full faulting by modifying the if()
condition, however there are two problems with this:
a) there may be a performance impact, since more loads and stores will be
emulated via the kernel
b) some extra implementation is needed in
arch/arm/mm/alignment.c:do_alignment_t32_to_handler() to recognise the 32-bit
Thumb-2 load and store instructions (since all processors which have these
instructions also have native unaligned access support); recognising these
instructions is not implemented at present, which means that such instructions
will either SIGILL, or spin as they are continuously refaulted.
** Changed in: linux-fsl-imx51 (Ubuntu)
Status: Fix Released => Confirmed
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CONFIG_NEON=y causes platform lockups with certain application/platform
combinations
https://bugs.launchpad.net/bugs/507416
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