Re: [RFC 21/21] arm/cpu-features: Document custom vcpu model

[Kashyap Chamarthy <kchamart@redhat.com>]

On Mon, Nov 04, 2024 at 04:30:17PM +0000, Peter Maydell wrote:
> On Mon, 4 Nov 2024 at 15:34, Eric Auger <eric.auger@redhat.com> wrote:

[...]

> > > If possible, it would be really helpful (and user-friendly) to be able
> > > to specify the CPU feature names as you see under /proc/cpuinfo, and be
> > > able to turn the flags on or off:
> > >
> > >         -M virt -cpu franken,rndr=on,ts=on,fhm=off
> > >
> > > (... instead of specifying long system register IDs that groups together
> > > a bunch of CPU features.  If I understand it correctly, the register
> > > "ID_AA64ISAR0_EL1" maps to a set of visible features listed here:
> > > https://docs.kernel.org/arch/arm64/cpu-feature-registers.html)
> > Not all the writable ID regs are visible through the above technique.
> > But indeed I think we converged on the idea to use higher level feature
> > names than ID reg field values.
> > However we need to study the feasibility and mappings between those high
> > level features and ID reg field values.
> > The cons is that we need to describe this mapping manually. Besides
> > being cumbersome this is also error prone.
> 
> You might be interested in "Arm Architecture Features" on
> https://developer.arm.com/Architectures/A-Profile%20Architecture#Downloads
> which includes a 1.8MB Features.json which is a machine
> readable version of the "what are the features and their
> dependencies and ID registers and so on" information.

I just took a quick at the JSON file.  It was a bit difficult to get a
sense of the structure of the file.  Maybe it makes more sense to those
who are more Arm-aware than me.  Or maybe that's what you meant by
"alpha quality" below :)

Out of curiosity, I tried to find the features for the register
"ID_AA64ISAR0_EL1".  I was navigating via trial-and-error with `jq`.
There's a lot of "right"/"left" traversing:

    $> jq '.parameters[136]|.constraints[1]|.right|.left|.value' Features.json 
    "FEAT_TME"

The register name is buried under this:

    $> jq '.parameters[136]|.constraints[1]|.right|.right|.left|.arguments' Features.json 
    [
      {
        "_type": "Types.Field",
        "value": {
          "field": "TME",
          "instance": null,
          "name": "ID_AA64ISAR0_EL1",
          "slices": null,
          "state": "AArch64"
        }
      }
    ]


I was niavely expecting a more predictable structure such as:

    $> jq '.register[ID_AA64ISAR0_EL1]|.fields' Features.json
    FEAT_RNG
    FEAT_TLBIOS
    ...

> But note that (a) it is alpha quality and (b) I am not personally
> going to try to interpret what might be reasonable to do with it
> based on the legal notice attached to it: that's a matter for
> you and your lawyer ;-)

Hmm, it sounds like until point (b) is clarified, this file is out of
consideration from an upstream point of view.

> -- PMM
> 

-- 
/kashyap