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

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Date:   Thu, 19 May 2022 11:13:54 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Heiko Carstens <hca@...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 5/19/22 07:37, Heiko Carstens wrote:
> 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?

I'd guess we looked into the wrong direction for auto-load.

We'll look into that for 5.20 but we'll take this patch with the "m" 
tristate for 5.19.

Thanks for the pointer and review.