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

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Date:   Tue, 12 Jul 2022 21:25:43 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Steffen Eiden <seiden@...ux.ibm.com>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, nrb@...ux.ibm.com
Subject: Re: [PATCH 1/3] s390/cpufeature: rework to allow more than only
 hwcap bits

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@...ux.ibm.com>
> Signed-off-by: Heiko Carstens <hca@...ux.ibm.com>
...
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright IBM Corp. 2022
> + * Author(s): Steffen Eiden <seiden@...ux.ibm.com>
> + *            Heiko Carstens <hca@...ux.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));