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

[Will Deacon <will@kernel.org>]

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