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Subject: Re: [RFC 1/1] s390/pci: expose UID checking state in sysfs
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Bjorn Helgaas <helgaas@kernel.org>, linux-pci@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        Pierre Morel <pmorel@linux.ibm.com>,
        Peter Oberparleiter <oberpar@linux.ibm.com>,
        Viktor Mihajlovski <mihajlov@linux.ibm.com>
References: <20210113185500.GA1918216@bjorn-Precision-5520>
 <675aa466-59ea-cf8a-6eec-caa6478ba4cd@linux.ibm.com>
 <20210114134453.bkfik4zjt5ehz6d5@wittgenstein> <YABOHuejsuriwSPn@kroah.com>
From:   Niklas Schnelle <schnelle@linux.ibm.com>
Message-ID: <a567c3d2-1dd2-6b33-8b1a-0a607b601ef8@linux.ibm.com>
Date:   Thu, 14 Jan 2021 16:06:11 +0100
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On 1/14/21 2:58 PM, Greg Kroah-Hartman wrote:
> On Thu, Jan 14, 2021 at 02:44:53PM +0100, Christian Brauner wrote:
>> On Thu, Jan 14, 2021 at 02:20:10PM +0100, Niklas Schnelle wrote:
>>>
>>>
>>> On 1/13/21 7:55 PM, Bjorn Helgaas wrote:
>>>> On Wed, Jan 13, 2021 at 08:47:58AM +0100, Niklas Schnelle wrote:
>>>>> On 1/12/21 10:50 PM, Bjorn Helgaas wrote:
>>>>>> On Mon, Jan 11, 2021 at 10:38:57AM +0100, Niklas Schnelle wrote:
>>>>>>> We use the UID of a zPCI adapter, or the UID of the function zero if
>>>>>>> there are multiple functions in an adapter, as PCI domain if and only if
>>>>>>> UID Checking is turned on.
>>>>>>> Otherwise we automatically generate domains as devices appear.
>>>>>>>
>>>>>>> The state of UID Checking is thus essential to know if the PCI domain
>>>>>>> will be stable, yet currently there is no way to access this information
>>>>>>> from userspace.
>>>>>>> So let's solve this by showing the state of UID checking as a sysfs
>>>>>>> attribute in /sys/bus/pci/uid_checking
>>>>
>>>>>>> +/* Global zPCI attributes */
>>>>>>> +static ssize_t uid_checking_show(struct kobject *kobj,
>>>>>>> +				 struct kobj_attribute *attr, char *buf)
>>>>>>> +{
>>>>>>> +	return sprintf(buf, "%i\n", zpci_unique_uid);
>>>>>>> +}
>>>>>>> +
>>>>>>> +static struct kobj_attribute sys_zpci_uid_checking_attr =
>>>>>>> +	__ATTR(uid_checking, 0444, uid_checking_show, NULL);
>>>>>>
>>>>>> Use DEVICE_ATTR_RO instead of __ATTR.
>>>>>
>>>>> It's my understanding that DEVICE_ATTR_* is only for
>>>>> per device attributes. This one is global for the entire
>>>>> Z PCI. I just tried with BUS_ATTR_RO instead
>>>>> and that works but only if I put the attribute at
>>>>> /sys/bus/pci/uid_checking instead of with a zpci
>>>>> subfolder. This path would work for us too, we
>>>>> currently don't have any other global attributes
>>>>> that we are planning to expose but those could of
>>>>> course come up in the future.
>>>>
>>>> Ah, I missed the fact that this is a kobj_attribute, not a
>>>> device_attribute.  Maybe KERNEL_ATTR_RO()?  Very few uses so far, but
>>>> seems like it might fit?
>>>>
>>>> Bjorn
>>>>
>>>
>>> KERNEL_ATTR_* is currently not exported in any header. After
>>> adding it to include/linuc/sysfs.h it indeed works perfectly.
>>> Adding Christian Brauner as suggested by get_maintainers for
>>> their opinion. I'm of course willing to provide a patch
>>
>> Hey Niklas et al. :)
>>
>> I think this will need input from Greg. He should be best versed in
>> sysfs attributes. The problem with KERNEL_ATTR_* to me seems that it's
>> supposed to be kernel internal. Now, that might just be a matter of
>> renaming the macro but let's see whether Greg has any better idea or
>> more questions. :)
> 
> The big question is, why are you needing this?
> 
> No driver or driver subsystem should EVER be messing with a "raw"
> kobject like this.  Just use the existing DEVICE_* macros instead
> please.
> 
> If you are using a raw kobject, please ask me how to do this properly,
> as that is something that should NEVER show up in the /sys/devices/*
> tree.  Otherwise userspace tools will break.
> 
> thanks,
> 
> greg k-h

Hi Greg,

this is for an architecture specific but global i.e. not device bound PCI
attribute. That's why DEVICE_ATTR_* does not work. BUS_ATTR_* would work
but only if we place the attribute directly under /sys/bus/pci/new_attr.

I'm aware that this is quite unusual in fact I couldn't find anything
similar. That's why this is an RFC, with a lengthy cover letter
explaining our use case, that I sent to Bjorn to figure out where to
even place the attribute.

So I guess this is indeed me asking you how to do this properly.
That said it does not show up under /sys/devices/* only /sys/bus/pci/*.

Best regards,
Niklas