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

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Message-ID: <4132ba2a-f5ad-25ba-7f74-72369b8a140b@linux.ibm.com>
Date:   Wed, 13 Jul 2022 10:32:21 +0200
From:   Steffen Eiden <seiden@...ux.ibm.com>
To:     Heiko Carstens <hca@...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 7/12/22 21:25, Heiko Carstens wrote:
> 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.

OK, makes sense.

> 
>> +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.
The loop in print_cpu_modalias() was the reason why I added all
current HWCAPs. The current implementation runs through all HWCAPs
using cpu_have_feature() and I feared that reducing to just MSA and
VXRS has effects in the reporting of CPU-features to userspace.

I double checked the output of 'grep features /proc/cpuinfo' and it
stays the same, for 5.19-rc6, 5.19-rc6+this series, 5.19-rc6+this series 
with just the two S390_CPU_FEATUREs. I might have misunderstood what 
happens in that loop in print_cpu_modalias().

Now that I think again over this piece of code my additions do not make
sense at all for me.

I will reduce that array again to the two explicitly needed entries.


> 
> 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));
I'll change it.