[PATCH 1/9] KVM: arm64: selftests: Add helpers to extract a field of an ID register

[Oliver Upton]

Hi Reiji,

On Wed, Aug 24, 2022 at 10:08:38PM -0700, Reiji Watanabe wrote:
> Introduce helpers to extract a field of an ID register.
> Subsequent patches will use those helpers.
> 
> Signed-off-by: Reiji Watanabe <reijiw at google.com>
> ---
>  .../selftests/kvm/include/aarch64/processor.h     |  2 ++
>  .../testing/selftests/kvm/lib/aarch64/processor.c | 15 +++++++++++++++
>  2 files changed, 17 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/include/aarch64/processor.h b/tools/testing/selftests/kvm/include/aarch64/processor.h
> index a8124f9dd68a..a9b4b4e0e592 100644
> --- a/tools/testing/selftests/kvm/include/aarch64/processor.h
> +++ b/tools/testing/selftests/kvm/include/aarch64/processor.h
> @@ -193,4 +193,6 @@ void smccc_hvc(uint32_t function_id, uint64_t arg0, uint64_t arg1,
>  
>  uint32_t guest_get_vcpuid(void);
>  
> +int cpuid_get_sfield(uint64_t val, int field_shift);
> +unsigned int cpuid_get_ufield(uint64_t val, int field_shift);
>  #endif /* SELFTEST_KVM_PROCESSOR_H */
> diff --git a/tools/testing/selftests/kvm/lib/aarch64/processor.c b/tools/testing/selftests/kvm/lib/aarch64/processor.c
> index 6f5551368944..0b2ad46e7ff5 100644
> --- a/tools/testing/selftests/kvm/lib/aarch64/processor.c
> +++ b/tools/testing/selftests/kvm/lib/aarch64/processor.c
> @@ -528,3 +528,18 @@ void smccc_hvc(uint32_t function_id, uint64_t arg0, uint64_t arg1,
>  		       [arg4] "r"(arg4), [arg5] "r"(arg5), [arg6] "r"(arg6)
>  		     : "x0", "x1", "x2", "x3", "x4", "x5", "x6", "x7");
>  }
> +
> +/* Helpers to get a signed/unsigned feature field from ID register value */
> +int cpuid_get_sfield(uint64_t val, int field_shift)
> +{
> +	int width = 4;
> +
> +	return (int64_t)(val << (64 - width - field_shift)) >> (64 - width);
> +}

I don't believe this helper is ever used.

> +unsigned int cpuid_get_ufield(uint64_t val, int field_shift)
> +{
> +	int width = 4;
> +
> +	return (uint64_t)(val << (64 - width - field_shift)) >> (64 - width);
> +}

I would recommend not open-coding this and instead make use of
ARM64_FEATURE_MASK(). You could pull in linux/bitfield.h to tools, or do
something like this:

  #define ARM64_FEATURE_GET(ftr, val)					\
  	  	  ((ARM64_FEATURE_MASK(ftr) & val) >> ftr##_SHIFT)

Slight preference for FIELD_{GET,SET}() as it matches the field
extraction in the kernel as well.

--
Thanks,
Oliver