Re: [PATCH 1/3] s390/cpufeature: rework to allow more than only hwcap bits

Heiko Carstens
2022-07-12T19:44:02Z

On Tue, Jul 12, 2022 at 12:52:18PM +0200, Steffen Eiden wrote:
> Rework cpufeature implementation to allow for various cpu feature
> indications, which is not only limited to hwcap bits. This is achieved
> by adding a sequential list of cpu feature numbers, where each of them
> is mapped to an entry which indicates what this number is about.
>
> Each entry contains a type member, which indicates what feature
> name space to look into (e.g. hwcap, or cpu facility). If wanted this
> allows also to automatically load modules only in e.g. z/VM
> configurations.
>
> Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
...
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright IBM Corp. 2022
> + * Author(s): Steffen Eiden <seiden@linux.ibm.com>
> + * Heiko Carstens <hca@linux.ibm.com>

Please don't add my name + email address in source code. I just
recently removed that everywhere since email addresses may change, and
git history is more than enough for me. It's up to you if you want to
keep your name + email address here.

> +static struct s390_cpu_feature s390_cpu_features[MAX_CPU_FEATURES] = {
> + [S390_CPU_FEATURE_ESAN3] = {.type = TYPE_HWCAP, .num = HWCAP_NR_ESAN3},
> + [S390_CPU_FEATURE_ZARCH] = {.type = TYPE_HWCAP, .num = HWCAP_NR_ZARCH},
> + [S390_CPU_FEATURE_STFLE] = {.type = TYPE_HWCAP, .num = HWCAP_NR_STFLE},
> + [S390_CPU_FEATURE_MSA] = {.type = TYPE_HWCAP, .num = HWCAP_NR_MSA},
> + [S390_CPU_FEATURE_LDISP] = {.type = TYPE_HWCAP, .num = HWCAP_NR_LDISP},
> + [S390_CPU_FEATURE_EIMM] = {.type = TYPE_HWCAP, .num = HWCAP_NR_EIMM},
> + [S390_CPU_FEATURE_DFP] = {.type = TYPE_HWCAP, .num = HWCAP_NR_DFP},
> + [S390_CPU_FEATURE_HPAGE] = {.type = TYPE_HWCAP, .num = HWCAP_NR_HPAGE},
> + [S390_CPU_FEATURE_ETF3EH] = {.type = TYPE_HWCAP, .num = HWCAP_NR_ETF3EH},
> + [S390_CPU_FEATURE_HIGH_GPRS] = {.type = TYPE_HWCAP, .num = HWCAP_NR_HIGH_GPRS},
> + [S390_CPU_FEATURE_TE] = {.type = TYPE_HWCAP, .num = HWCAP_NR_TE},
> + [S390_CPU_FEATURE_VXRS] = {.type = TYPE_HWCAP, .num = HWCAP_NR_VXRS},
> + [S390_CPU_FEATURE_VXRS_BCD] = {.type = TYPE_HWCAP, .num = HWCAP_NR_VXRS_BCD},
> + [S390_CPU_FEATURE_VXRS_EXT] = {.type = TYPE_HWCAP, .num = HWCAP_NR_VXRS_EXT},
> + [S390_CPU_FEATURE_GS] = {.type = TYPE_HWCAP, .num = HWCAP_NR_GS},
> + [S390_CPU_FEATURE_VXRS_EXT2] = {.type = TYPE_HWCAP, .num = HWCAP_NR_VXRS_EXT2},
> + [S390_CPU_FEATURE_VXRS_PDE] = {.type = TYPE_HWCAP, .num = HWCAP_NR_VXRS_PDE},
> + [S390_CPU_FEATURE_SORT] = {.type = TYPE_HWCAP, .num = HWCAP_NR_SORT},
> + [S390_CPU_FEATURE_DFLT] = {.type = TYPE_HWCAP, .num = HWCAP_NR_DFLT},
> + [S390_CPU_FEATURE_VXRS_PDE2] = {.type = TYPE_HWCAP, .num = HWCAP_NR_VXRS_PDE2},
> + [S390_CPU_FEATURE_NNPA] = {.type = TYPE_HWCAP, .num = HWCAP_NR_NNPA},
> + [S390_CPU_FEATURE_PCI_MIO] = {.type = TYPE_HWCAP, .num = HWCAP_NR_PCI_MIO},
> + [S390_CPU_FEATURE_SIE] = {.type = TYPE_HWCAP, .num = HWCAP_NR_SIE},
> +};

I only realized now that you added all HWCAP bits here. It was
intentional that I added only the two bits which are currently used
for several reasons:

- Keep the array as small as possible.
- No need to keep this array in sync with HWCAPs, if new ones are added.
- There is a for loop in print_cpu_modalias() which iterates over all
MAX_CPU_FEATURES entries; this should be as fast as possible. Adding
extra entries burns cycles for no added value.

Any future user which requires a not yet listed feature, can simply
add it when needed.

> +int cpu_have_feature(unsigned int num)
> +{
> + struct s390_cpu_feature *feature;
> +
> + feature = &s390_cpu_features[num];
> + switch (feature->type) {
> + case TYPE_HWCAP:
> + return !!(elf_hwcap & (1UL << feature->num));

Before somebody else mentions it, I could have done better. Nowadays
this should be:

return !!(elf_hwcap & BIT(feature->num));