From c97cba4ea44910df1f7b1af5dba79013fb44c383 Mon Sep 17 00:00:00 2001 From: Soby Mathew Date: Wed, 25 Sep 2019 14:03:41 +0100 Subject: Fix the CAS spinlock implementation 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 Signed-off-by: Olivier Deprez --- docs/design/firmware-design.rst | 7 +++++-- docs/getting_started/user-guide.rst | 4 ++++ 2 files changed, 9 insertions(+), 2 deletions(-) (limited to 'docs') diff --git a/docs/design/firmware-design.rst b/docs/design/firmware-design.rst index dc082082..2cbd9c94 100644 --- a/docs/design/firmware-design.rst +++ b/docs/design/firmware-design.rst @@ -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 ~~~~~~~~~ diff --git a/docs/getting_started/user-guide.rst b/docs/getting_started/user-guide.rst index 44bfb7a3..ae276f25 100644 --- a/docs/getting_started/user-guide.rst +++ b/docs/getting_started/user-guide.rst @@ -820,6 +820,10 @@ Common build options reduces SRAM usage. Refer to `Library at ROM`_ for further details. Default is 0. +- ``USE_SPINLOCK_CAS``: Setting this build flag to 1 selects the spinlock + implementation variant using the ARMv8.1-LSE compare-and-swap instruction. + Notice this option is experimental and only available to AArch64 builds. + - ``V``: Verbose build. If assigned anything other than 0, the build commands are printed. Default is 0. -- cgit v1.2.3