Re: [PATCH v3 07/10] perf: arm_spe: Add support for filtering on data source

[James Clark]

On 17/07/2025 3:29 pm, Will Deacon wrote:
> On Tue, Jul 15, 2025 at 02:04:18PM +0100, James Clark wrote:
> > On 14/07/2025 3:04 pm, Will Deacon wrote:
> > > On Thu, Jun 05, 2025 at 11:49:05AM +0100, James Clark wrote:
> > > > @@ -406,6 +416,9 @@ static u64 arm_spe_event_to_pmsfcr(struct perf_event *event)
> > > >    	if (ATTR_CFG_GET_FLD(attr, inv_event_filter))
> > > >    		reg |= PMSFCR_EL1_FnE;
> > > > +	if (ATTR_CFG_GET_FLD(attr, data_src_filter))
> > > > +		reg |= PMSFCR_EL1_FDS;
> > >
> > > Is the polarity correct here? The description of PMSDSFR_EL1.S<m> suggests
> > > that setting bits to 1 _excludes_ the FDS filtering.
> >
> > Setting filter bits to 1 means that samples matching are included. Setting
> > bits to 0 means that they are excluded. And PMSFCR_EL1.FDS enables filtering
> > as a whole, so if the user sets any filter bit to 1 we want to enable
> > filtering:
> >
> >    PMSDSFR_EL1.S<m>
> >
> >    0b0  If PMSFCR_EL1.FDS is 1, do not record load operations that have
> >         bits [5:0] of the Data Source packet set to <m>.
> >
> >    0b1  Load operations with Data Source <m> are unaffected by
> >         PMSFCR_EL1.FDS.
> >
> > I think it's all the right way around and it ends up being the same as the
> > other filters in SPE. Because we're using any bit being set to enable the
> > filtering, the only thing you can't do is enable filtering with a 0 filter,
> > but I didn't think that was useful. See the previous discussion on this
> > here:
> > https://lore.kernel.org/all/5752f039-51c1-4452-b5df-03ff06da7be3@xxxxxxxxxx/
> >
> > Reading the "Data source filtering" section in the docs change at the end
> > might help too.
>
> Sorry, but I still don't get it :/
>
> afaict, if any of the bits in 'data_src_filter' are _zero_ then we
> should set PMSFCR_EL1.FDS. That also means that a mask of zero means all
> loads are filtered, which is what the architecture says and is what we
> should provide to userspace.
>
> Will

We'd have to add another format flag to enable data source filtering 
then, because otherwise the default would be zero and people's samples 
would disappear.

But the only use cases I could think of were more like "I want to see 
samples from data source 1":

  -e arm_spe/data_src_filter=0x1/

Or "I want to see all data sources except 1":

  -e arm_spe/data_src_filter=0xfffffffe/

Filtering out all samples with any data source didn't seem to make sense 
to me, and I think you can already do that with the other filters 
(remove loads etc).

It would be a shame to be inconsistent and to add an enable flag just 
for that one case because the other filters in SPE are auto enabled for 
non-zero values. Although to be fair for PMSFCR.FT and others, zero 
filters are explicitly not allowed:

  If this field is set to 1 and the PMSFCR_EL1.{ST, LD, B} bits are all
  set to zero, it is CONSTRAINED UNPREDICTABLE whether no samples are
  recorded or the PE behaves as if PMSFCR_EL1.FT is set to 0

Seems like FDS doesn't end up as neat as the others, but IMO I can't see 
anyone needing a zero filter. I did discuss it with Leo and we decided 
that we could always add the enable flag at a later date if a use case 
turned up and it wouldn't be a breaking change.

But if you think it's there so it should be exposed I can add it.

James