[webkit-dev] Data Memory Barrier ARMv6 question

[Caio Lima]

2017-07-05 12:41 GMT-03:00 JF Bastien <jfbastien at apple.com>:
> On Linux you can do the following:
>
> ((void(*)())0xffff0fa0)();
>
>
> That address contains a helper which does the “right” barrier, including if
> you’re not on an SMP system it’ll do nothing.
>
> Details:
> https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt
> That file also lists other Linux helpers.
>
> I think for ARMv6 it makes sense to use these helpers. AFAIK the mcr barrier
> instruction ins’t supported by all ARMv6 CPUs.

Hi JF. Do you mind point me where the mcr isn't supported by all ARMv6
CPU? I've found in ARMv6-M manual
(http://infocenter.arm.com/help/topic/com.arm.doc.ddi0419d/DDI0419D_armv6m_arm.pdf)
that it doesn't support coprocessor operations, but this architecture
is used by microcontroller chips.

Regards,
Caio.

> For ARMv7 and later, DMB ish is the right thing.
>
>
> On Jul 3, 2017, at 17:19, Caio Lima <ticaiolima at gmail.com> wrote:
>
> Hi all.
>
> I'm working in this patch
> (https://bugs.webkit.org/show_bug.cgi?id=172767) and Mark Lam raised
> some questions about the data memory barrier (DMB for short) in ARMv6
> using "mcr 15 ...". The point is that we are having divergences in ARM
> official reference manual about the semantics of this instruction. We
> have it discussed in the bug above and I would like to know if there
> is somebody with stronger ARM background that could help us there and
> then approve the patch to be committed.
>
> I thanks in advance and best regards,
> Caio Lima.
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