linux-kernel - Re: [PATCH v4 1/2] drivers/s390/char: Add Ultravisor io device

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Date:   Thu, 19 May 2022 07:37:16 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>
Cc:     Steffen Eiden <seiden@...ux.ibm.com>, Greg KH <greg@...ah.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Shuah Khan <shuah@...nel.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Nico Boehr <nrb@...ux.ibm.com>, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v4 1/2] drivers/s390/char: Add Ultravisor io device

On Wed, May 18, 2022 at 03:45:27PM +0200, Janosch Frank wrote:
> > > +	  The device is only available if the Ultravisor
> > > +	  Facility (158) is present.
> > 
> > Is there a reason why this is default "y"? If you think this should be
> > compiled into the kernel if used, then why allow to make it a module
> > at all?
> > Instead you could get rid of a couple if lines of code.
> 
> There was a lot of discussion around this already and the "Y" was chosen as
> auto-loading this is a pain and therefore the SCLP and CHSC-Misc set it to Y
> and we took that as an example (Steffen spoke to Peter to get guidance).
> 
> I'm sure that we want the possibility to have this as a module. Personally
> I'd choose "m" over "y" since the module is only useful for a very small
> amount of users.

Why not simply use module_cpu_feature_match() to implement auto module
loading like we do it for the crypto modules? That would require that
either the uv facility is represented within elf hwcaps, or
alternatively the s390 implementation of cpu_feature() needs to be
changed to work with cpu facilities instead of hwcap bits.
(see arch/s390/include/asm/cpufeature.h)

This doesn't look too difficult. Or was there a reason not to go this route?