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

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Message-ID: <48550162-0f8c-4b23-dea4-b9060b24eed9@linux.ibm.com>
Date:   Wed, 18 May 2022 15:45:27 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>,
        Steffen Eiden <seiden@...ux.ibm.com>
Cc:     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 5/18/22 13:47, Heiko Carstens wrote:
> On Tue, May 10, 2022 at 02:47:23PM +0000, Steffen Eiden wrote:
>> diff --git a/drivers/s390/char/Kconfig b/drivers/s390/char/Kconfig
>> index 6cc4b19acf85..e9b9902abbaf 100644
>> --- a/drivers/s390/char/Kconfig
>> +++ b/drivers/s390/char/Kconfig
>> @@ -100,6 +100,16 @@ config SCLP_OFB
>>   	  This option enables the Open-for-Business interface to the s390
>>   	  Service Element.
>>   
>> +config S390_UV_UAPI
>> +	def_tristate y
>> +	prompt "Ultravisor userspace API"
>> +	help
>> +	  Selecting exposes parts of the UV interface to userspace
>> +	  by providing a misc character device at /dev/uv.
>> +	  Using IOCTLs one can interact with the UV.
>> +	  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.