Re: [PATCH 2/2] target/riscv/tcg-cpu.c: add extension properties for all cpus

[Alistair Francis]

On Fri, Sep 29, 2023 at 8:39 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Tue, Sep 26, 2023 at 03:31:09PM -0300, Daniel Henrique Barboza wrote:
> > At this moment we do not expose extension properties for vendor CPUs
> > because that would allow users to change them via command line. The
> > drawback is that if we were to add an API that shows all CPU properties,
> > e.g. qmp-query-cpu-model-expansion, we won't be able to show extensions
> > state of vendor CPUs.
> >
> > We have the required machinery to create extension properties for vendor
> > CPUs while not allowing users to enable extensions. Disabling existing
> > extensions is allowed since it can be useful for debugging.
> >
> > Change the set() callback cpu_set_multi_ext_cfg() to allow enabling
> > extensions only for generic CPUs. In cpu_add_multi_ext_prop() let's not
> > set the default values for the properties if we're not dealing with
> > generic CPUs, otherwise the values set in cpu_init() of vendor CPUs will
> > be overwritten. And finally, in tcg_cpu_instance_init(), add cpu user
> > properties for all CPUs.
> >
> > For the veyron-v1 CPU, we're now able to disable existing extensions
> > like smstateen:
> >
> > $ ./build/qemu-system-riscv64 --nographic -M virt \
> >     -cpu veyron-v1,smstateen=false
> >
> > But setting extensions that the CPU didn't set during cpu_init(), like
> > V, is not allowed:
> >
> > $ ./build/qemu-system-riscv64 --nographic -M virt \
> >     -cpu veyron-v1,v=true
> > qemu-system-riscv64: can't apply global veyron-v1-riscv-cpu.v=true:
> >   'veyron-v1' CPU does not allow enabling extensions
>
> Why should we block the user if they want to enable an extra
> feature, over and above what is built-in to the CPU model ?

It ends up being tricky to maintain. On top of that we can report a
specific vendor CPU to guests, but then we don't correctly model it
(as extensions might be disabled).

Alistair

> Is there some technical reason that prevents this from working ?
>
> With regards,
> Daniel
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