> On 5/13/26 6:33 PM, Shaju Abraham wrote:
>> Add a declarative description of every architecturally defined field in the
>> Arm64 ID registers in cpu-idregs.h.inc file. This file will be included
>> multiple times with different definitions to build the per-field arch values,
>> per-register field descriptor arrays, arm-idregs table etc. The
>> architecturely
>> defined values for each ID register field is extracted from kernel's
>> arch/arm64/tools/sysreg file. The safe-value tags and default values are
> In the past we were rather advised to use AARCHMRS instead of sysreg.
> This is what is currently done in [PATCH v4 00/17] kvm/arm: Introduce a
> customizable aarch64 KVM host model.
I am not sure if we can get safe rules from AARCHMRS. Otherwise it’s fine.
>> derived from the kernel's ftr_bits array. Aarch32 ID registers are added
>> with dummy field so they can be defaulted to 0 by later commits.
>>
>> X-macro structure:
>> IDREG_START(REG)
>> IDREG_FIELD_START(REG, FIELD, SHIFT, LENGTH, SAFE_RULE, DEFAULT_VAL)
> Why do you need SAFE_RULE/DEFAULT_VAL? As far as I understand this is
> used when we have different flavours of CPUS in SMP and we face
> mismatches between CPU features. Why do we care here?
>
> I don't see safe_rule used anywhere.
> I see arm_idregs_reset_to_defaults() uses the default value though.
> Should we use the Registers.json reset value instead?
Kernel uses safe_rules to show a common denominator of cpu features in SMP and
same rules are
used by KVM to verify if certain field value is valid to show to guest on the
given host.
KVM_SET_ONE_REG validates the writable fields with the SAFE_RULE defined in
ftr_bits table.
So, SAFE_RULE will be used in v2 to validate idregs even before we writeback to
the KVM, and to
pass supported field values for a property on a given host to management stack
(libvirt). Something like
feat_AES supports (off, aes, pmull) on this host.
Registers.json can probably be used for default_vals, but I will need to check
further on this.
In short: default_vals -> show no feature.
safe_rule -> valid values for guest given host values.
> taking FHM field if I compare with sysreg I have:
> UnsignedEnum 51:48 FHM
> 0b0000 NI
> 0b0001 IMP
> 0b0010 F16F32DOT
> 0b0011 F16F32MM
> So why do we miss some values?
The table was created based on sysreg file in 6.18.
A script added the arch values. Register, Fields, safe values and default
values are
maintained manually.
Warm Regards,
Khushit
