Re: [PATCH 9/9] arm64: Documentation - Expose CPU feature registers

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Suzuki,

On Thu, Nov 24, 2016 at 01:40:09PM +0000, Suzuki K. Poulose wrote:
> --- /dev/null
> +++ b/Documentation/arm64/cpu-feature-registers.txt
> @@ -0,0 +1,198 @@
> +		ARM64 CPU Feature Registers
> +		===========================
> +
> +Author: Suzuki K Poulose <suzuki.poulose@arm.com>
> +
> +
> +This file describes the API for exporting the AArch64 CPU ID/feature
> +registers to userspace. The availability of this API is advertised
> +via the HWCAP_CPUID in HWCAPs.

s/API/ABI/ maybe?

> +
> +1. Motivation
> +---------------
> +
> +The ARM architecture defines a set of feature registers, which describe
> +the capabilities of the CPU/system. Access to these system registers is
> +restricted from EL0 and there is no reliable way for an application to
> +extract this information to make better decisions at runtime. There is
> +limited information available to the application via HWCAPs, however
> +there are some issues with their usage.
> +
> + a) Any change to the HWCAPs requires an update to userspace (e.g libc)
> +    to detect the new changes, which can take a long time to appear in
> +    distributions. Exposing the registers allows applications to get the
> +    information without requiring updates to the toolchains.
> +
> + b) Access to HWCAPs is sometimes limited (e.g prior to libc, or
> +    when ld is initialised at startup time).
> +
> + c) HWCAPs cannot represent non-boolean information effectively. The
> +    architecture defines a canonical format for representing features
> +    in the ID registers; this is well defined and is capable of
> +    representing all valid architecture variations. Exposing the ID
> +    registers avoids having to come up with HWCAP representations and
> +    parsing code.

For point (c) above, we don't (yet?) have an actual case on AArch64
where HWCAP needs more than a boolean value.

And just to clarify my position: I consider that we should continue to
expose HWCAP for new features (e.g. SVE) in parallel with the CPUID
access emulation. There are different use-cases for them (i.e. dynamic
loader uses HWCAP for the ifunc resolver).

> +3. Implementation
> +--------------------
> +
> +The infrastructure is built on the emulation of the 'MRS' instruction.
> +Accessing a restricted system register from an application generates an
> +exception and ends up in SIGILL being delivered to the process.
> +The infrastructure hooks into the exception handler and emulates the
> +operation if the source belongs to the supported system register space.
> +
> +The infrastructure emulates only the following system register space:
> +	Op0=3, Op1=0, CRn=0
> +
> +(See Table C5-6 'System instruction encodings for non-Debug System
> +register accesses' in ARMv8 ARM DDI 0487A.h, for the list of
> +registers).
> +
> +
> +The following rules are applied to the value returned by the
> +infrastructure:
> +
> + a) The value of an 'IMPLEMENTATION DEFINED' field is set to 0.
> + b) The value of a reserved field is populated with the reserved
> +    value as defined by the architecture.
> + c) The value of a field marked as not 'visible', is set to indicate
> +    the feature is missing (as defined by the architecture).

I don't understand point (c) above. If it is marked as not 'visible', it
is always reported to user as 0. The above could be misinterpreted as
reporting missing architecture features.

[...]
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 94c188f..fb331de 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -81,6 +81,10 @@ static bool __maybe_unused
>  cpufeature_pan_not_uao(const struct arm64_cpu_capabilities *entry, int __unused);
>  
>  
> +/*
> + * NOTE: Any changes to the visibility of features should be kept in
> + * sync with the documentation of the CPU feature register API.

s/API/ABI/

-- 
Catalin