Re: [PATCH] arm64: errata: Add Ampere erratum AC04_CPU_50 workaround alternative

[Marc Zyngier <maz@kernel.org>]

On Mon, 27 Jan 2025 20:18:29 +0000,
Zaid Alali <zaidal@os.amperecomputing.com> wrote:
> 
> Add an alternative code sequence to work around Ampere erratum
> AC03_CPU_50 on AmpereOne and Ampere1A.
> 
> Due to AC03_CPU_50, when ICC_PMR_EL1 should have a value of 0xf0 a
> direct read of the register will return a value of 0xf8. An incorrect
> value from a direct read can only happen with the value 0xf0.

Under which precise conditions? Does it equally apply to virtual
interrupts or SCR_EL3.FIQ==0, for which there is no non-secure shift
(which I can only assume is the source of the erratum)? Does it
equally affect G0 and G1 interrupts?

> 
> Note: Currently there are no checks against a value of 0xf0, and that
> save restore of 0xf8 -> 0xf0 is fine, so this is all future proofing.
> 
> Signed-off-by: Zaid Alali <zaidal@os.amperecomputing.com>
> ---
>  arch/arm64/Kconfig                  | 16 ++++++++++++++++
>  arch/arm64/include/asm/arch_gicv3.h |  2 +-
>  arch/arm64/include/asm/daifflags.h  |  4 ++--
>  arch/arm64/include/asm/irqflags.h   |  6 +++---
>  arch/arm64/include/asm/sysreg.h     |  9 +++++++++
>  arch/arm64/kernel/cpu_errata.c      | 15 +++++++++++++++
>  arch/arm64/kernel/entry.S           |  4 ++++
>  arch/arm64/tools/cpucaps            |  1 +

Please add an entry to Documentation/arch/arm64/silicon-errata.txt.

>  8 files changed, 51 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index fcdd0ed3eca8..8d6e263d66c7 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -461,6 +461,22 @@ config AMPERE_ERRATUM_AC03_CPU_38
>  
>  	  If unsure, say Y.
>  
> +config AMPERE_ERRATUM_AC03_CPU_50
> +	bool "AmpereOne: AC03_CPU_50: Certain checks for ICC_PMR_EL1 that expects the value 0xf0 may read 0xf8 instead"
> +	default y
> +	help
> +	  This option adds an alternative code sequence to work around Ampere
> +	  erratum AC03_CPU_50 on AmpereOne and Ampere1A.
> +
> +	  Due to AC03_CPU_50, when ICC_PMR_EL1 should have a value of 0xf0 a
> +	  direct read of the register will return a value of 0xf8. An incorrect
> +	  value from a direct read can only happen with the value 0xf0.
> +
> +	  The workaround for the erratum will do logical AND 0xf0 to the
> +	  value read from ICC_PMR_EL1 register before returning the value.
> +
> +	  If unsure, say Y.
> +

An alternative for this would simply to prevent the enabling of pNMI
on this platform.

	M.

-- 
Without deviation from the norm, progress is not possible.