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

Will Deacon
2025-07-17T16:06:40Z

On Thu, Jul 17, 2025 at 04:16:32PM +0100, James Clark wrote:
>
>
> 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@linaro.org/
> > >
> > > 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.

What about if we expose the inverse of PMSDSFR_EL1 to userspace instead?

Will