linux-kernel - Re: [PATCH v9 3/3] s390x: KVM: resetting the Topology-Change-Report

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Message-ID: <5de8d8c2-100d-f935-667c-1090ee31277d@linux.ibm.com>
Date:   Wed, 18 May 2022 18:55:26 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, borntraeger@...ibm.com,
        frankja@...ux.ibm.com, cohuck@...hat.com, thuth@...hat.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com, wintera@...ux.ibm.com,
        seiden@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v9 3/3] s390x: KVM: resetting the Topology-Change-Report



On 5/18/22 16:33, David Hildenbrand wrote:
> On 16.05.22 16:21, Pierre Morel wrote:
>>
>>
>> On 5/12/22 12:01, David Hildenbrand wrote:
>>>>>
>>>>> I think we prefer something like u16 when copying to user space.
>>>>
>>>> but then userspace also has to expect a u16, right?
>>>
>>> Yep.
>>>
>>
>> Yes but in fact, inspired by previous discussion I had on the VFIO
>> interface, that is the reason why I did prefer an int.
>> It is much simpler than a u16 and the definition of a bit.
>>
>> Despite a bit in a u16 is what the s3990 achitecture proposes I thought
>> we could make it easier on the KVM/QEMU interface.
>>
>> But if the discussion stops here, I will do as you both propose change
>> to u16 in KVM and userland and add the documentation for the interface.
> 
> In general, we pass via the ABI fixed-sized values -- u8, u16, u32, u64
> ... instead of int. Simply because sizeof(int) is in theory variable
> (e.g., 32bit vs 64bit).
> 
> Take a look at arch/s390/include/uapi/asm/kvm.h and you won't find any
> usage of int or bool.
> 
> Having that said, I'll let the maintainers decide. Using e.g., u8 is
> just the natural thing to do on a Linux ABI, but we don't really support
> 32 bit ... maybe we'll support 128bit at one point? ;)
> 

OK then I use u16 with a flag in case we get something in the utilities 
which is related to the topology in the future.

Thanks,
Pierre

-- 
Pierre Morel
IBM Lab Boeblingen