Re: [PATCH v5 3/3] perf x86: Exposing an Uncore unit to PMON for Intel Xeon® server platform

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Feb 12, 2020 at 03:58:50PM -0500, Liang, Kan wrote:
> 
> 
> On 2/12/2020 12:31 PM, Sudarikov, Roman wrote:
> > On 11.02.2020 23:14, Greg KH wrote:
> > > On Tue, Feb 11, 2020 at 02:59:21PM -0500, Liang, Kan wrote:
> > > > 
> > > > On 2/11/2020 1:57 PM, Greg KH wrote:
> > > > > On Tue, Feb 11, 2020 at 10:42:00AM -0800, Andi Kleen wrote:
> > > > > > On Tue, Feb 11, 2020 at 09:15:44AM -0800, Greg KH wrote:
> > > > > > > On Tue, Feb 11, 2020 at 07:15:49PM +0300,
> > > > > > > roman.sudarikov@linux.intel.com wrote:
> > > > > > > > +static ssize_t skx_iio_mapping_show(struct device *dev,
> > > > > > > > +                struct device_attribute *attr, char *buf)
> > > > > > > > +{
> > > > > > > > +    struct pmu *pmu = dev_get_drvdata(dev);
> > > > > > > > +    struct intel_uncore_pmu *uncore_pmu =
> > > > > > > > +        container_of(pmu, struct intel_uncore_pmu, pmu);
> > > > > > > > +
> > > > > > > > +    struct dev_ext_attribute *ea =
> > > > > > > > +        container_of(attr, struct dev_ext_attribute, attr);
> > > > > > > > +    long die = (long)ea->var;
> > > > > > > > +
> > > > > > > > +    return sprintf(buf, "0000:%02x\n",
> > > > > > > > skx_iio_stack(uncore_pmu, die));
> > > > > > > If "0000:" is always the "prefix" of the output of
> > > > > > > this file, why have
> > > > > > > it at all as you always know it is there?
> > > > 
> > > > I think Roman only test with BIOS configured as single-segment. So he
> > > > hard-code the segment# here.
> > > > 
> > > > I'm not sure if Roman can do some test with multiple-segment
> > > > BIOS. If not, I
> > > > think we should at least print a warning here.
> > > > 
> > > > > > > What is ever going to cause that to change?
> > > > > > I think it's just to make it a complete PCI address.
> > > > > Is that what this really is?  If so, it's not a "complete" pci address,
> > > > > is it?  If it is, use the real pci address please.
> > > > I think we don't need a complete PCI address here. The attr is
> > > > to disclose
> > > > the mapping information between die and PCI BUS. Segment:BUS
> > > > should be good
> > > > enough.
> > > "good enough" for today, but note that you can not change the format of
> > > the data in the file in the future, you would have to create a new file.
> > > So I suggest at least try to future-proof it as much as possible if you
> > > _know_ this could change.
> > > 
> > > Just use the full pci address, there's no reason not to, otherwise it's
> > > just confusing.
> > > 
> > > thanks,
> > > 
> > > greg k-h
> > Hi Greg,
> > 
> > Yes, the "Segment:Bus" pair is enough to distinguish between different
> > Root ports.
> 
> I think Greg suggests us to use full PCI address here.
> 
> Hi Greg,
> 
> There may be several devices are connected to IIO stack. There is no full
> PCI address for IIO stack.

Please define "full" for me.  Please please don't tell me you are just
using a truncated version of the PCI address.  I thought we got rid of
all of that nonsense 10 years ago...

> I don't think we can list all of devices in the same IIO stack with full PCI
> address here either. It's not necessary, and only increase maintenance
> overhead.

Then what exactly _IS_ this number, if not the PCI address?

Something made up to look almost like a PCI address, but not quite?
Somethine else?

> I think we may have two options here.
> 
> Option 1: Roman's proposal.The format of the file is "Segment:Bus". For the
> future I can see, the format doesn't need to be changed.
>     E.g. $ls /sys/devices/uncore_<type>_<pmu_idx>/die0
>          $0000:7f

Again, fake PCI address?

> Option 2: Use full PCI address, but use -1 to indicate invalid address.
>     E.g. $ls /sys/devices/uncore_<type>_<pmu_idx>/die0
>          $0000:7f:-1:-1

"Invalid"?  Why?  Why not just refer to the 0:0 device, as that's the
bus "root" address (or whatever it's called, I can't remember PCI stuff
all that well...)

> Should we use the format in option 2?

What could userspace do with a -1 -1 address?

thanks,

greg k-h