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

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Date:   Tue, 11 Feb 2020 12:14:27 -0800
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     Andi Kleen <ak@...ux.intel.com>, roman.sudarikov@...ux.intel.com,
        peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org,
        linux-kernel@...r.kernel.org, eranian@...gle.com,
        bgregg@...flix.com, alexander.antonov@...el.com
Subject: Re: [PATCH v5 3/3] perf x86: Exposing an Uncore unit to PMON for Intel Xeon® server platform

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@...ux.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