Re: [PATCH 15/19] linux-user/arm: Replace ARM_FEATURE_VFP* tests for HWCAP

[Peter Maydell]

On Fri, 14 Feb 2020 at 18:16, Richard Henderson
<address@hidden> wrote:
>
> Use isar feature tests instead of feature bit tests.
>
> Although none of QEMUs current cpus have VFPv3 without D32,
> replace the large comment explaining why with one line that
> sets ARM_HWCAP_ARM_VFPv3D16 under the correct conditions.
> Mirror the test sequence used in the linux kernel.
>
> Signed-off-by: Richard Henderson <address@hidden>
> ---
>  linux-user/elfload.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index f3080a1635..c52c814a2e 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -468,22 +468,28 @@ static uint32_t get_elf_hwcap(void)
>
>      /* EDSP is in v5TE and above, but all our v5 CPUs are v5TE */
>      GET_FEATURE(ARM_FEATURE_V5, ARM_HWCAP_ARM_EDSP);
> -    GET_FEATURE(ARM_FEATURE_VFP, ARM_HWCAP_ARM_VFP);
>      GET_FEATURE(ARM_FEATURE_IWMMXT, ARM_HWCAP_ARM_IWMMXT);
>      GET_FEATURE(ARM_FEATURE_THUMB2EE, ARM_HWCAP_ARM_THUMBEE);
>      GET_FEATURE(ARM_FEATURE_NEON, ARM_HWCAP_ARM_NEON);
> -    GET_FEATURE(ARM_FEATURE_VFP3, ARM_HWCAP_ARM_VFPv3);
>      GET_FEATURE(ARM_FEATURE_V6K, ARM_HWCAP_ARM_TLS);
> -    GET_FEATURE(ARM_FEATURE_VFP4, ARM_HWCAP_ARM_VFPv4);
> +    GET_FEATURE(ARM_FEATURE_LPAE, ARM_HWCAP_ARM_LPAE);
> +
>      GET_FEATURE_ID(arm_div, ARM_HWCAP_ARM_IDIVA);
>      GET_FEATURE_ID(thumb_div, ARM_HWCAP_ARM_IDIVT);
> -    /* All QEMU's VFPv3 CPUs have 32 registers, see VFP_DREG in translate.c.
> -     * Note that the ARM_HWCAP_ARM_VFPv3D16 bit is always the inverse of
> -     * ARM_HWCAP_ARM_VFPD32 (and so always clear for QEMU); it is unrelated
> -     * to our VFP_FP16 feature bit.
> +    /*
> +     * Note that none of QEMU's cpus have double precision without single
> +     * precision support in VFP, so only test the single precision field.
>       */

Why not actually mirror the kernel's test sequence, rather
than having a comment about how we deviate from it ?

> -    GET_FEATURE(ARM_FEATURE_VFP3, ARM_HWCAP_ARM_VFPD32);
> -    GET_FEATURE(ARM_FEATURE_LPAE, ARM_HWCAP_ARM_LPAE);
> +    GET_FEATURE_ID(aa32_fpsp_v2, ARM_HWCAP_ARM_VFP);
> +    if (cpu_isar_feature(aa32_fpsp_v3, cpu)) {
> +        hwcaps |= ARM_HWCAP_ARM_VFPv3;
> +        if (cpu_isar_feature(aa32_simd_r32, cpu)) {
> +            hwcaps |= ARM_HWCAP_ARM_VFPD32;
> +        } else {
> +            hwcaps |= ARM_HWCAP_ARM_VFPv3D16;
> +        }
> +    }
> +    GET_FEATURE_ID(aa32_simdfmac, ARM_HWCAP_ARM_VFPv4);
>
>      return hwcaps;
>  }

thanks
-- PMM