[cacao] RFC: CACAO PR157 ARM memory barrier patch

[Robert Lougher]

On 11 March 2011 15:55, Andrew Haley <aph at redhat.com> wrote:
> On 03/11/2011 03:48 PM, Robert Lougher wrote:
>>
>> This is how I define MBARRIER for ARM in JamVM:
>>
>> #ifdef __ARM_ARCH_7A__
>> #define MBARRIER() __asm__ __volatile__ ("dmb" ::: "memory")
>> #else
>> #define MBARRIER() __asm__ __volatile__ ("" ::: "memory")
>> #endif
>
> But that's wrong for GNU/Linux binaries, surely.
>

Ubuntu defaults to ARMv7 and Thumb2, so this is OK for Ubuntu.  But
yes, I believe Debian builds binaries for ARMv4t.  I can understand
this in the past, as older binaries would run with no problem on later
chips.  But ARMv7 needs proper memory barriers, so I don't think the
Debian policy makes any sense here.  Using the kernel helper like this
just seems nasty to me (defining a function pointer via __kernel_dmb
(*(__kernel_dmb_t *) 0xffff0fa0)).  My opinion is that if you build
for ARMv4t, don't expect it to run on ARMv7.

Rob.

> Andrew.
>