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Fix the CAS spinlock implementation

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Make the spinlock implementation use ARMv8.1-LSE CAS instruction based
on a platform build option. The CAS-based implementation used to be
unconditionally selected for all ARM8.1+ platforms.

The previous CAS spinlock implementation had a bug wherein the spin_unlock()
implementation had an `sev` after `stlr` which is not sufficient. A dsb is
needed to ensure that the stlr completes prior to the sev. Having a dsb is
heavyweight and a better solution would be to use load exclusive semantics
to monitor the lock and wake up from wfe when a store happens to the lock.
The patch implements the same.

Change-Id: I5283ce4a889376e4cc01d1b9d09afa8229a2e522
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
This commit is contained in:
Soby Mathew 2019-09-25 14:03:41 +01:00 • committed by Olivier Deprez
parent ace23683be
commit c97cba4ea4
5 changed files with 43 additions and 37 deletions
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7
docs/design/firmware-design.rst
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@ -2540,8 +2540,11 @@ Armv8.1-A
This Architecture Extension is targeted when ``ARM_ARCH_MAJOR`` >= 8, or when
``ARM_ARCH_MAJOR`` == 8 and ``ARM_ARCH_MINOR`` >= 1.
- The Compare and Swap instruction is used to implement spinlocks. Otherwise,
the load-/store-exclusive instruction pair is used.
- By default, a load-/store-exclusive instruction pair is used to implement
spinlocks. The ``USE_SPINLOCK_CAS`` build option when set to 1 selects the
spinlock implementation using the ARMv8.1-LSE Compare and Swap instruction.
Notice this instruction is only available in AArch64 execution state, so
the option is only available to AArch64 builds.
Armv8.2-A
~~~~~~~~~