Re: [PATCH v6 06/23] xen/riscv: introduce guest riscv,isa string

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.07.2026 17:59, Oleksii Kurochko wrote:
> Introduce build_guest_isa_str() to generate the riscv,isa string to be
> passed to the guest via the Device Tree riscv,isa property.
> 
> Introduce the per-domain guest ISA bitmap, populated during domain
> creation by calling init_guest_isa().
> 
> Introduce struct riscv_isa_ext_entry with a new guest_supported field
> to filter out ISA extensions that should not be exposed to guests:
> 
> - f/d/q/v: FPU and vector context save/restore are not yet implemented
>   for guests.
> - Z*inx are not exposed either: they aren't in riscv_isa_ext[], so they
>   can never be set in riscv_isa and thus never reach a guest, and no
>   current hardware/guest-OS advertises or expects them. Supporting them
>   would be cheaper than F/D/Q (FP values stay in integer registers Xen
>   already context-switches), but is left as future work.
> - h: Nested virtualisation is not supported.
> - sstc: Xen owns the supervisor timer; guests must use SBI.
> - svade: Xen manages hardware A/D bit updates in stage-2 page tables.
> - svpbmt: Page-based memory types are not yet wired up in stage-2 code.
> 
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

In principle
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

But see below.

> ---
> Changes in v6:
> - build_guest_isa_str() now takes a `const struct domain *d` instead of a
>   raw `const unsigned long *isa_bitmap`, to leave room for using more than
>   just the bitmap in the future.
> - Compute the guest-visible ISA bitmap once at boot, into a new
>   __ro_after_init `guest_isa` bitmap (compute_guest_isa(), called at the
>   end of riscv_fill_hwcap()), instead of re-deriving it from
>   riscv_isa_ext[] on every domain creation in init_guest_isa(). All guests
>   currently get the same extension set, so this avoids repeating
>   identical work per domain; will need revisiting if/when per-domain ISA
>   policy is introduced.
> - struct arch_domain's `isa` field is now `const unsigned long *isa`
>   instead of an embedded bitmap; init_guest_isa() just points it at the
>   shared `guest_isa` bitmap rather than copying bits into a per-domain
>   array.
> - Mark riscv_isa_ext[] __initconstrel, since its entries hold name pointers
>   and the need for relocations requires that the compiler emit the data to
>   a writable section.
> - Make build_guest_isa_str() __init as it is called during make_cpus_node()
>   which is used only (at least, for now) in build time of domain.

This last item is confusing. For one as there's no call site being added here
in the first place. And then - why would the string need re-calculating when
the bitmap is calculated once at start of day?

Jan