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

[James Clark <james.clark@linaro.org>]

On 15/07/2025 1:57 pm, Will Deacon wrote:
> 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

I'll give it a go but that could end up being fragile to something that 
is dependent on two different ID registers in the future. Then we'd end 
up in the same situation for a different reason.