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    Subject[PATCH AUTOSEL 4.19 064/101] arm64: assembler: Switch ESB-instruction with a vanilla nop if !ARM64_HAS_RAS
    Date
    From: James Morse <james.morse@arm.com>

    [ Upstream commit 2b68a2a963a157f024c67c0697b16f5f792c8a35 ]

    The ESB-instruction is a nop on CPUs that don't implement the RAS
    extensions. This lets us use it in places like the vectors without
    having to use alternatives.

    If someone disables CONFIG_ARM64_RAS_EXTN, this instruction still has
    its RAS extensions behaviour, but we no longer read DISR_EL1 as this
    register does depend on alternatives.

    This could go wrong if we want to synchronize an SError from a KVM
    guest. On a CPU that has the RAS extensions, but the KConfig option
    was disabled, we consume the pending SError with no chance of ever
    reading it.

    Hide the ESB-instruction behind the CONFIG_ARM64_RAS_EXTN option,
    outputting a regular nop if the feature has been disabled.

    Reported-by: Julien Thierry <julien.thierry@arm.com>
    Signed-off-by: James Morse <james.morse@arm.com>
    Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/arm64/include/asm/assembler.h | 4 ++++
    1 file changed, 4 insertions(+)

    diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
    index f90f5d83b228..5a97ac853168 100644
    --- a/arch/arm64/include/asm/assembler.h
    +++ b/arch/arm64/include/asm/assembler.h
    @@ -112,7 +112,11 @@
    * RAS Error Synchronization barrier
    */
    .macro esb
    +#ifdef CONFIG_ARM64_RAS_EXTN
    hint #16
    +#else
    + nop
    +#endif
    .endm

    /*
    --
    2.20.1
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