Re: [PATCH 5/5] perf/x86: Relax privilege filter restriction on AMD IBS

[Namhyung Kim <namhyung@kernel.org>]

On Tue, Oct 15, 2024 at 07:06:24PM +0530, Ravi Bangoria wrote:
> Hi Namhyung,
> 
> On 05-Sep-24 8:40 AM, Namhyung Kim wrote:
> > While IBS is available for per-thread profiling, still regular users
> > cannot open an event due to the default paranoid setting (2) which
> > doesn't allow unprivileged users to get kernel samples.  That means
> > it needs to set exclude_kernel bit in the attribute but IBS driver
> > would reject it since it has PERF_PMU_CAP_NO_EXCLUDE.  This is not what
> > we want and I've been getting requests to fix this issue.
> > 
> > This should be done in the hardware, but until we get the HW fix we may
> > allow exclude_{kernel,user} in the attribute and silently drop the
> > samples in the PMU IRQ handler.  It won't guarantee the sampling
> > frequency or even it'd miss some with fixed period too.  Not ideal,
> > but that'd still be helpful to regular users.
> > 
> > To minimize the confusion, let's add 'swfilt' bit to attr.config2 which
> > is exposed in the sysfs format directory so that users can figure out
> > if the kernel support the privilege filters by software.
> > 
> >   $ perf record -e ibs_op/swfilt=1/uh true
> 
> Shall we add an example in tools/perf/Documentation/perf-amd-ibs.txt?

Yep, I'll update the document when I send the tooling changes.

> 
> 
> > +static struct attribute *swfilt_attrs[] = {
> > +	&format_attr_swfilt.attr,
> > +	NULL,
> > +};
> > +
> >  static struct attribute *fetch_l3missonly_attrs[] = {
> >  	&fetch_l3missonly.attr.attr,
> >  	NULL,
> > @@ -598,6 +604,11 @@ static struct attribute_group group_rand_en = {
> >  	.attrs = rand_en_attrs,
> >  };
> >  
> > +static struct attribute_group group_swfilt = {
> > +	.name = "format",
> > +	.attrs = swfilt_attrs,
> > +};
> > +
> >  static struct attribute_group group_fetch_l3missonly = {
> >  	.name = "format",
> >  	.attrs = fetch_l3missonly_attrs,
> > @@ -612,6 +623,7 @@ static struct attribute_group group_zen4_ibs_extensions = {
> >  
> >  static const struct attribute_group *fetch_attr_groups[] = {
> >  	&group_rand_en,
> > +	&group_swfilt,
> >  	&empty_caps_group,
> >  	NULL,
> >  };
> 
> Causes:
> 
>   # dmesg
>   sysfs: cannot create duplicate filename '/devices/ibs_fetch/format'
>   Failed to register pmu: ibs_fetch, reason -17
> 
> Rename rand_en_attrs[] to fetch_attrs[], add &format_attr_swfilt.attr
> to it and remove &group_swfilt from fetch_attr_groups[]. And I guess
> it should work.

Thanks for the review, I'll update the code as you suggested.

Namhyung