[PATCH 4/4] ARM: hwcap: disable HWCAP_SWP if the CPU advertises it has exclusives

Catalin Marinas catalin.marinas at arm.com
Mon Jul 7 10:50:58 PDT 2014

On Mon, Jul 07, 2014 at 05:31:43PM +0100, Russell King - ARM Linux wrote:
> On Mon, Jul 07, 2014 at 04:59:42PM +0100, Catalin Marinas wrote:
> > Going on topic again, LDREXB is present in ARM1136 r1 but your changes
> > would not detect it (and it's a different ID_ISAR3 format anyway). If we
> > care about this case, you could either pretend that ARM1136 r1 is ARMv6K
> > and link those decisions to v6K+ tests or check for individual features
> > which for ARM1136 would require a different check from standard CPUID.
> Ever since ARMv6 was first added, HWCAP_SWP has been indicated for all
> ARMv6 CPUs and their derivatives.  Indeed, the SWP instruction is
> supported in hardware.
> Hence, indicating HWCAP_SWP on ARMv6 whether or not there is LDREXB
> support is /not/ incorrect in any way.
> ARM1136 r1 added LDREXB as part of the ARMv6K adoption, but it kept
> the SWP instruction (which can't be disabled.)  So, indicating
> HWCAP_SWP also is /not/ incorrect.

Fine be me.

> However, when it comes to SMP, we would _prefer_ userspace to use
> the exclusives instructions, though it seems that the SWP behaviour
> on an ARMv6K SMP system with respect to other CPUs is not documented.
> All things being equal, we would prefer SWP not be used there because
> we don't know whether it's safe.  Unfortunately, we can't turn the
> instruction off, so we can't emulate it.

IIRC, ARM11MPCore is fine. Certain ARMv7 MP implementations have issues
with SWP, though not all AFAIK.

> However, we know that some userspace running there may well contain
> SWP instructions.  So, as there aren't any bug reports, it's probably
> safe to assume that SWP is safe on ARMv6K SMP systems.
> So, the upshot of this is:
> * SWP on all ARMv6 is fine.
> * Advertising HWCAP_SWP on all ARMv6 is fine too.
> * We would prefer not to advertise HWCAP_SWP on ARMv6 with support
>   for the exclusives, and we can do that trivially where we can
>   check the CPUID values.
> * ARM1136 is either ARMv6 or ARMv6K depending on its revision -
>   and thus may have LDREXB, but LDREXB is not trivially detectable.
> I believe what I'm doing in this patch is architecturally correct, I
> also believe it to be safe for the troublesome ARM1136 by way of
> defaulting to advertising HWCAP_SWP.

I agree. With the above assumptions, feel free to add my ack for the
whole series.

Acked-by: Catalin Marinas <catalin.marinas at arm.com>

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