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Date:   Thu, 14 Jan 2021 16:17:06 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Niklas Schnelle <schnelle@linux.ibm.com>
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>
Subject: Re: [RFC 1/1] s390/pci: expose UID checking state in sysfs
Message-ID: <YABgcnWPLJxYlWUR@kroah.com>
References: <20210113185500.GA1918216@bjorn-Precision-5520>
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 <20210114134453.bkfik4zjt5ehz6d5@wittgenstein>
 <YABOHuejsuriwSPn@kroah.com>
 <a567c3d2-1dd2-6b33-8b1a-0a607b601ef8@linux.ibm.com>
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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 :)

Where _exactly_ are you wanting to put this attribute?

> 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 :)

thanks,

greg k-h