Re: [PATCH 1/2] arm64: errata: Work around AmpereOne's erratum AC03_CPU_36

Oliver Upton
2025-04-15T17:12:34Z

On Tue, Apr 15, 2025 at 08:47:10AM -0700, D Scott Phillips wrote:
> AC03_CPU_36 can cause asynchronous exceptions to be routed to the wrong
> exception level if an async exception coincides with an update to the
> controls for the target exception level in HCR_EL2. On affected
> machines, always do writes to HCR_EL2 with async exceptions blocked.
>
> Signed-off-by: D Scott Phillips <scott@os.amperecomputing.com>
> ---
> arch/arm64/Kconfig | 17 +++++++++++++++++
> arch/arm64/include/asm/sysreg.h | 18 ++++++++++++++++--
> arch/arm64/kernel/cpu_errata.c | 14 ++++++++++++++
> arch/arm64/tools/cpucaps | 1 +
> 4 files changed, 48 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index a182295e6f08b..e5fd87446a3b8 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -445,6 +445,23 @@ menu "Kernel Features"
>
> menu "ARM errata workarounds via the alternatives framework"
>
> +config AMPERE_ERRATUM_AC03_CPU_36
> + bool "AmpereOne: AC03_CPU_36: CPU can take an invalid exception, if an asynchronous exception to EL2 occurs while EL2 software is changing the EL2 exception controls."
> + default y
> + help
> + This option adds an alternative code sequence to work around Ampere
> + errata AC03_CPU_36 on AmpereOne.
> +
> + If an async exception happens at the same time as an update to the
> + controls for the target EL for async exceptions, an exception can be
> + delivered to the wrong EL. For example, an EL may be routed from EL2
> + to EL1.
> +
> + The workaround masks all asynchronous exception types when writing
> + to HCR_EL2.
> +
> + If unsure, say Y.
> +
> config AMPERE_ERRATUM_AC03_CPU_38
> bool "AmpereOne: AC03_CPU_38: Certain bits in the Virtualization Translation Control Register and Translation Control Registers do not follow RES0 semantics"
> default y
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 2639d3633073d..e7781f7e7f7a7 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -1136,14 +1136,28 @@
> __val; \
> })
>
> +#define __sysreg_is_hcr_el2(r) \
> + (__builtin_strcmp("hcr_el2", __stringify(r)) == 0)

This looks fragile. What about:

write_sysreg(hcr, HCR_EL2);

or:

write_sysreg_s(hcr, SYS_HCR_EL2);

Thanks,
Oliver