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

From: Heiko Carstens
Date: Thu Jul 14 2022 - 06:52:39 EST

> > > +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.