On Fri, 17 Jun 2011, Paul Brook wrote:
There is still going to be a small cost even in hardware fixup so this is very much worth solving despite it's "becoming invisible" because the chips are hiding / solving it already.
But I believe that h/w feature is turned off in Linux by default. You have to add noalign to the kernel command line to enable.
I think you'll find the hardware fixups are enabled by default on CPUs with that design, quite possibly it can't be turned off.
The situation is much more complicated than that. There are two completely different models for misaligned accesses (v6 and pre-v6). v7 cores are only required to support the former.
However even under the v6 model some instructions will fault on misaligned addresses, and the CPU may be configured to fault many others. The exact behavior depends on the particular instruction chosen by the compiler. I don't know whether Linux currently knows how to enable alignment checking on v6/v7 hardware.
It does. Has done for a long time. Please see arch/arm/mm/alignment.c:
static int __init alignment_init(void) { [...] /* * ARMv6 and later CPUs can perform unaligned accesses for * most single load and store instructions up to word size. * LDM, STM, LDRD and STRD still need to be handled. * * Ignoring the alignment fault is not an option on these * CPUs since we spin re-faulting the instruction without * making any progress. */ 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; }
And if you look at arch/arm/mm/proc-v6.S in __v6_setup, you'll find out that the CR_U bit is set.
int main(int argc, char * argv[]) {
char buf[8]; void *v = &buf[1]; unsigned int *p = (unsigned int *)v;This does not (reliably) do what you expect. The compiler need not align buf.
Printing the value of p should clarify this.
And, as we can see above, the "simple" accesses are left to the hardware to fix up. However, if the misaligned access is performed using a 64-bit value pointer, then the kernel will trap an exception and the access will be simulated.
Nicolas