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

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Date:   Thu, 14 Jul 2022 12:52:25 +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

> > > +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},
...
> > 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().

It is used on cpu hotplug to generate a MODALIAS environment
variable. You can check that by running "udevadm monitor -p"
and then switching a cpu off/on.

This environment variable is then used by systemd/udev to load
feature matching modules via kmod.