Re: [PATCH v3 08/10] arm/cpu: more customization for the kvm host cpu model

[Cornelia Huck <cohuck@redhat.com>]

On Tue, May 13 2025, Daniel P. Berrangé <berrange@redhat.com> wrote:

> On Mon, Apr 14, 2025 at 06:38:47PM +0200, Cornelia Huck wrote:
>> From: Eric Auger <eric.auger@redhat.com>
>> 
>> If the interface for writable ID registers is available, expose uint64
>> SYSREG properties for writable ID reg fields exposed by the host
>> kernel. Properties are named  SYSREG_<REG>_<FIELD> with REG and FIELD
>> being those used  in linux arch/arm64/tools/sysreg. This done by
>> matching the writable fields retrieved from the host kernel against the
>> generated description of sysregs.
>> 
>> An example of invocation is:
>> -cpu host,SYSREG_ID_AA64ISAR0_EL1_DP=0x0
>> which sets DP field of ID_AA64ISAR0_EL1 to 0.
>
> For the value you are illustrating 0x0 - is this implying that
> all the flags take an arbitrary integer hex value ?
>
> This would be different from x86, where CPU feature flags are
> a boolean on/off state.

Most of the fields are 4 bits, the allowed values vary (there are also
some fields that are single bits, or wider.) The FEAT_xxx values (which
can be expressed via ID register fields, or a combination thereof) are
mostly boolean, but there are also some of them that can take values.

We could cook up pseudo-features that are always on/off, but I don't
like that approach: they would be QEMU only, whereas the ID register
fields and FEAT_xxx features are all defined in the Arm documentation.

An additional difference from x86 would be that FEAT_xxx featues are not
neccessarily configurable (only if the host kernel supports changing the
ID register field(s) backing the feature.)