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Subject: Re: [PATCH v2 05/15] s390: vfio-ap: base implementation of VFIO AP
 device driver
To:     Cornelia Huck <cohuck@redhat.com>
Cc:     Pierre Morel <pmorel@linux.vnet.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, freude@de.ibm.com, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com, borntraeger@de.ibm.com,
        kwankhede@nvidia.com, bjsdjshi@linux.vnet.ibm.com,
        pbonzini@redhat.com, alex.williamson@redhat.com,
        alifm@linux.vnet.ibm.com, mjrosato@linux.vnet.ibm.com,
        jjherne@linux.vnet.ibm.com, thuth@redhat.com,
        pasic@linux.vnet.ibm.com, fiuczy@linux.vnet.ibm.com,
        buendgen@de.ibm.com
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 <c7ae5d8e-aa40-9da5-afdf-af53487de3c3@linux.vnet.ibm.com>
 <20180228191019.20507888.cohuck@redhat.com>
From:   Tony Krowiak <akrowiak@linux.vnet.ibm.com>
Date:   Wed, 28 Feb 2018 15:34:32 -0500
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On 02/28/2018 01:10 PM, Cornelia Huck wrote:
> On Wed, 28 Feb 2018 11:43:37 -0500
> Tony Krowiak <akrowiak@linux.vnet.ibm.com> wrote:
>
>> On 02/28/2018 10:33 AM, Pierre Morel wrote:
>>> On 27/02/2018 15:28, Tony Krowiak wrote:
> (...)
>
>>>> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
>>>> index cbe1d97..9f23caf 100644
>>>> --- a/arch/s390/Kconfig
>>>> +++ b/arch/s390/Kconfig
>>>> @@ -771,6 +771,14 @@ config VFIO_CCW
>>>>          To compile this driver as a module, choose M here: the
>>>>          module will be called vfio_ccw.
>>>>
>>>> +config VFIO_AP
>>>> +    def_tristate m
> Any reason you default to m instead of n here?
None in particular, is there a good reason to change this to n?
>
>>>> +    prompt "Support for virtual Adjunct Processor device interface"
>>> The VFIO AP devices are not virtual.
>>> What about
>>>      "VFIO support for AP devices"
>> Sounds good.
> +1
>
>>>   
>>>> +    depends on ZCRYPT && VFIO_MDEV_DEVICE
>>>> +    help
>>>> +        driver grants access to Adjunct Processor (AP) devices
> s/driver/This driver/
Okay, I'll make the change
>
>>>> +        via the VFIO mediated device interface.
> You also might want to add
>
> "To compile this driver as a module, choose M here: the
> module will be called..."
Will do
>
>>>> +
>>>>    endmenu
> It's a tad confusing to find this in the I/O submenu, but I don't
> really have a better idea.
I wasn't sure either, but couldn't think of a better location. Anybody
else have any ideas?
>
>>>>    menu "Dump support"
>>>> diff --git a/arch/s390/configs/default_defconfig
>>>> b/arch/s390/configs/default_defconfig
>>>> index 5af8458..40fa3f6 100644
>>>> --- a/arch/s390/configs/default_defconfig
>>>> +++ b/arch/s390/configs/default_defconfig
>>> Not sure that this file belongs to this patch
>> Neither am I, but at the time I inserted this here - well before August
>> of last year - I was using vfio-ccw as a model.
>> If someone can verify this does not belong here, I'd be more than happy
>> to remove it.
> I don't see any entry for VFIO_CCW in there?
There isn't now, but there was at the time I first started working on this.
My first pass was a lot of cut-and-paste followed by modification of the
vfio_ccw implementation, but that was long ago. This is a remnant of that
time. Like I said, I'd be happy to remove this.
>
>>>   
>>>> @@ -719,3 +719,6 @@ CONFIG_APPLDATA_BASE=y
>>>>    CONFIG_KVM=m
>>>>    CONFIG_KVM_S390_UCONTROL=y
>>>>    CONFIG_VHOST_NET=m
>>>> +VFIO_MDEV=m
>>>> +VFIO_MDEV_DEVICE=m
>>>> +CONFIG_VFIO_AP=m
>>> What is your goal when modifying this three files?
>>> Could you add a comment in the commit message?
>> As stated above, this was originally based on the vfio-ccw model and has
>> been in the
>> patch series since its inception. I'd be happy to remove it if it is not
>> necessary.
> I'd vote for removing it.
Consider them gone.
>
> (...)
>
>>>> +static int vfio_ap_matrix_dev_create(void)
>>>> +{
>>>> +    int ret;
>>>> +
>>>> +    vfio_ap_root_device = root_device_register(VFIO_AP_ROOT_NAME);
>>>> +
>>>> +    ret = PTR_ERR_OR_ZERO(vfio_ap_root_device);
>>> IS_ERR() is enough, root_device_register() never return NULL.
>> I searched the kernel code to look at other places the
>> root_device_register()
>> function is called to see how the return value is handled. I've seen all
>> of the
>> following used:
>> if (IS_ERR())
>>       ret = PTR_ERR()
>> PTR_ERR()
>> PTR_ERR_OR_ZERO()
>>
>> I'm not sure why this is a concern, but I'll use the first option above
>> since PTR_ERR_OR_ZERO() also embeds the first option.
> PTR_ERR_OR_ZERO() seems like the best choice for the way the return
> code is processed here. (It's just unfortunate that its name conjures
> up connotations of NULL-pointer handling.)
I changed it to:

     ret = IS_ERR(vfio_ap_root_device);
     if (ret) {
         ret = PTR_ERR(vfio_ap_root_device);
         goto done;
     }

Hopefully everybody is happy with this.
>
>>>> +    if (ret)
>>>> +        goto done;