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

[Leo Yan <leo.yan@arm.com>]

On Wed, May 21, 2025 at 09:54:48AM +0100, James Clark wrote:
> On 20/05/2025 5:22 pm, Leo Yan wrote:

[...]

> I'm thinking I'd rather leave it consistent with PMSFCR_EL1.FT and
> automatically enable PMSFCR_EL1.FDS for any non zero data-source filter.

This is fine for me.

Just a minor thing, for the case PMSDSFR_EL1 = 0xFFFF,FFFF,FFFF,FFFF,
we might consider to clear the PMSFCR_EL1.FDS bit.  This would be a bit
performance benefit for disabling data source filter rather than
enabling the filter with unaffecting all data sources.

> This means we don't need a tool change to set some other flag when a filter
> is provided (even if it's zero) and it's much simpler. It also doesn't
> prevent the possibility of adding the enable flag in the future if someone
> comes out with a need for it, but I don't think it needs to be done now.

The question comes down to the complexity in user-space tools.

Perf initializes the attribute configs to zeros. If we want to set all
bits in config4 as a default value, we would need additional change
in the perf tool. Also initializing config4 to all ones is likely to
cause confusion if other tools want to enable the feature.

I agree that a cleaner way would be to use an enable flag + mask, we can
defer to add flag if needed.

> TBH I can't imagine a case where someone would want to filter out any samples
> that have any data source. Surely you'd only be looking for a selected set
> of data sources, or no filtering at all.

Agreed this is a rare case.

Thanks,
Leo