Re: [PATCH v3 1/9] arm64: Repaint ID_AA64MMFR2_EL1.IDS description

[Ben Horgan <ben.horgan@arm.com>]

Hi Marc,

On 12/4/25 10:48, Marc Zyngier wrote:
> On Thu, 04 Dec 2025 10:36:54 +0000,
> Ben Horgan <ben.horgan@arm.com> wrote:
>>
>> Hi Marc,
>>
>> On 12/4/25 09:47, Marc Zyngier wrote:
>>> ID_AA64MMFR2_EL1.IDS, as described in the sysreg file, is pretty horrible
>>> as it diesctly give the ESR value. Repaint it using the usual NI/IMP
>>> identifiers to describe the absence/presence of FEAT_IDST.
>>>
>>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>>> ---
>>>  arch/arm64/kvm/hyp/nvhe/sys_regs.c | 2 +-
>>>  arch/arm64/tools/sysreg            | 4 ++--
>>>  2 files changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/arm64/kvm/hyp/nvhe/sys_regs.c b/arch/arm64/kvm/hyp/nvhe/sys_regs.c
>>> index 82da9b03692d4..107d62921b168 100644
>>> --- a/arch/arm64/kvm/hyp/nvhe/sys_regs.c
>>> +++ b/arch/arm64/kvm/hyp/nvhe/sys_regs.c
>>> @@ -134,7 +134,7 @@ static const struct pvm_ftr_bits pvmid_aa64mmfr2[] = {
>>>  	MAX_FEAT(ID_AA64MMFR2_EL1, UAO, IMP),
>>>  	MAX_FEAT(ID_AA64MMFR2_EL1, IESB, IMP),
>>>  	MAX_FEAT(ID_AA64MMFR2_EL1, AT, IMP),
>>> -	MAX_FEAT_ENUM(ID_AA64MMFR2_EL1, IDS, 0x18),
>>> +	MAX_FEAT_ENUM(ID_AA64MMFR2_EL1, IDS, IMP),
>>>  	MAX_FEAT(ID_AA64MMFR2_EL1, TTL, IMP),
>>>  	MAX_FEAT(ID_AA64MMFR2_EL1, BBM, 2),
>>>  	MAX_FEAT(ID_AA64MMFR2_EL1, E0PD, IMP),
>>> diff --git a/arch/arm64/tools/sysreg b/arch/arm64/tools/sysreg
>>> index 1c6cdf9d54bba..3261e8791ac03 100644
>>> --- a/arch/arm64/tools/sysreg
>>> +++ b/arch/arm64/tools/sysreg
>>> @@ -2257,8 +2257,8 @@ UnsignedEnum	43:40	FWB
>>>  	0b0001	IMP
>>>  EndEnum
>>>  Enum	39:36	IDS
>>
>> Should this also be changed to an UnsignedEnum?
> 
> I'm not sure this brings much when you only have two values. If IDS
> was growing a third value, and that there was an actual order in the
> numbering scheme, then yes, that'd be useful.

Joey just pointed out to me that there is a new third field. Not in the
arm reference manual yet, but mentioned in the xml. I'm unsure if it's
necessary to consider this at the moment though.

https://developer.arm.com/documentation/ddi0601/2025-09/AArch64-Registers/ID-AA64MMFR2-EL1--AArch64-Memory-Model-Feature-Register-2?lang=en

> 
> But at this stage, I'm not confident that this is desirable, let alone
> necessary.
> 
> Thanks,
> 
> 	M.
> 

Thanks,

Ben