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Subject: Re: [RFC 1/1] s390/pci: expose UID checking state in sysfs
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        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>
 <a567c3d2-1dd2-6b33-8b1a-0a607b601ef8@linux.ibm.com>
 <YABgcnWPLJxYlWUR@kroah.com>
From:   Niklas Schnelle <schnelle@linux.ibm.com>
Message-ID: <f63d5de5-6a31-8839-72ce-c6e937f91d4a@linux.ibm.com>
Date:   Thu, 14 Jan 2021 16:51:17 +0100
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On 1/14/21 4:17 PM, Greg Kroah-Hartman wrote:
> On Thu, Jan 14, 2021 at 04:06:11PM +0100, Niklas Schnelle wrote:
>>
>>
>> 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.
> 
> Then you are doing something wrong :)

That is very possible.

> 
> Where _exactly_ are you wanting to put this attribute?

I'm trying for /sys/bus/pci/zpci/uid_checking, I'm using
the below code and the attribute even shows up but reading
it gives me two 0 bytes only.
The relevant code is only a slight alteration of the original patch
as follows:

static ssize_t uid_checking_show(struct bus_type *bus, char *buf)
{
	return sprintf(buf, "%i\n", zpci_unique_uid);
}
static BUS_ATTR_RO(uid_checking);

static struct kset *zpci_global_kset;

static struct attribute *zpci_attrs_global[] = {
	&bus_attr_uid_checking.attr,
	NULL,
};

static struct attribute_group zpci_attr_group_global = {
	.attrs = zpci_attrs_global,
};

int __init zpci_sysfs_init(void)
{
	struct kset *pci_bus_kset;

	pci_bus_kset = bus_get_kset(&pci_bus_type);

	zpci_global_kset = kset_create_and_add("zpci", NULL, &pci_bus_kset->kobj);
	if (!zpci_global_kset)
		return -ENOMEM;

	return sysfs_create_group(&zpci_global_kset->kobj, &zpci_attr_group_global);
}


> 
>> 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/*.
> 
> Do NOT put random kobjects under a bus subsystem.  If you need that,
> then use BUS_ATTR_* as that is what it is there for.
> 
> Again, if you are in a driver subsystem, do not use a raw kobject.
> Either something is already there for you, or what you want to do is not
> correct :)

Understood and thanks for the clear advice!

> 
> thanks,
> 
> greg k-h
>