Re: [PATCH v3 04/10] arm64/boot: Enable EL2 requirements for SPE_FEAT_FDS

[Will Deacon <will@kernel.org>]

On Tue, Jul 15, 2025 at 01:48:03PM +0100, James Clark wrote:
> 
> 
> On 14/07/2025 2:54 pm, Will Deacon wrote:
> > On Thu, Jun 05, 2025 at 11:49:02AM +0100, James Clark wrote:
> > > SPE data source filtering (optional from Armv8.8) requires that traps to
> > > the filter register PMSDSFR be disabled. Document the requirements and
> > > disable the traps if the feature is present.
> > > 
> > > Tested-by: Leo Yan <leo.yan@arm.com>
> > > Signed-off-by: James Clark <james.clark@linaro.org>
> > > ---
> > >   Documentation/arch/arm64/booting.rst | 11 +++++++++++
> > >   arch/arm64/include/asm/el2_setup.h   | 14 ++++++++++++++
> > >   2 files changed, 25 insertions(+)
> > > 
> > > diff --git a/Documentation/arch/arm64/booting.rst b/Documentation/arch/arm64/booting.rst
> > > index dee7b6de864f..abd75085a239 100644
> > > --- a/Documentation/arch/arm64/booting.rst
> > > +++ b/Documentation/arch/arm64/booting.rst
> > > @@ -404,6 +404,17 @@ Before jumping into the kernel, the following conditions must be met:
> > >       - HDFGWTR2_EL2.nPMICFILTR_EL0 (bit 3) must be initialised to 0b1.
> > >       - HDFGWTR2_EL2.nPMUACR_EL1 (bit 4) must be initialised to 0b1.
> > > +  For CPUs with SPE data source filtering (FEAT_SPE_FDS):
> > > +
> > > +  - If EL3 is present:
> > > +
> > > +    - MDCR_EL3.EnPMS3 (bit 42) must be initialised to 0b1.
> > > +
> > > +  - If the kernel is entered at EL1 and EL2 is present:
> > > +
> > > +    - HDFGRTR2_EL2.nPMSDSFR_EL1 (bit 19) must be initialised to 0b1.
> > > +    - HDFGWTR2_EL2.nPMSDSFR_EL1 (bit 19) must be initialised to 0b1.
> > > +
> > >     For CPUs with Memory Copy and Memory Set instructions (FEAT_MOPS):
> > >     - If the kernel is entered at EL1 and EL2 is present:
> > > diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
> > > index 1e7c7475e43f..02b4a7fc016e 100644
> > > --- a/arch/arm64/include/asm/el2_setup.h
> > > +++ b/arch/arm64/include/asm/el2_setup.h
> > > @@ -279,6 +279,20 @@
> > >   	orr	x0, x0, #HDFGRTR2_EL2_nPMICFILTR_EL0
> > >   	orr	x0, x0, #HDFGRTR2_EL2_nPMUACR_EL1
> > >   .Lskip_pmuv3p9_\@:
> > > +	mrs	x1, id_aa64dfr0_el1
> > > +	ubfx	x1, x1, #ID_AA64DFR0_EL1_PMSVer_SHIFT, #4
> > > +	/* If SPE is implemented, */
> > > +	cmp	x1, #ID_AA64DFR0_EL1_PMSVer_IMP
> > > +	b.lt	.Lskip_spefds_\@
> > > +	/* we can read PMSIDR and */
> > > +	mrs_s	x1, SYS_PMSIDR_EL1
> > > +	and	x1, x1,  #PMSIDR_EL1_FDS
> > > +	/* if FEAT_SPE_FDS is implemented, */
> > > +	cbz	x1, .Lskip_spefds_\@
> > > +	/* disable traps to PMSDSFR. */
> > > +	orr	x0, x0, #HDFGRTR2_EL2_nPMSDSFR_EL1
> > 
> > Why is this being done here rather than alongside the existing SPE
> > configuration of HDFGRTR_EL2 and HDFGWTR_EL2 near the start of
> > __init_el2_fgt?
> > 
> I thought everything was separated by which trap configs it writes to,
> rather than the feature. This SPE feature is in HDFGRTR2 so I put it in
> __init_el2_fgt2 rather than __init_el2_fgt.

That's fair; __init_el2_fgt isn't the right place. But the redundancy of
re-reading PMSVer from DFR0 is a little jarring.

> I suppose we could have a single __init_el2_spe that writes to both HDFGRTR
> and HDFGRTR2 but we'd have to be careful to not overwrite what was already
> done in the other sections.

Right, perhaps it would be clearer to have trap-preserving macros for
features in a specific ID register rather than per-trap configuration
register macros.

In other words, we have something like __init_fgt_aa64dfr0 which would
configure the FGT and FGT2 registers based on features in aa64dfr0. I
think you'd need to have a play to see how it ends up looking but the
main thing to avoid is having duplicate ID register parsing code for
setting up FGT and FGT2 traps.

Will