Re: [PATCH v3 09/20] xen/dom0less: abstract Arm-specific p2m type name for device MMIO mappings

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.07.2025 17:58, Oleksii Kurochko wrote:
> Rename `p2m_mmio_direct_dev` to a more architecture-neutral alias
> `p2m_mmio_direct` to avoid leaking Arm-specific naming into common Xen code,
> such as dom0less passthrough property handling.
> 
> This helps reduce platform-specific terminology in shared logic and
> improves clarity for future non-Arm ports (e.g. RISC-V or PowerPC).
> 
> No functional changes — the definition is preserved via a macro alias
> for Arm.
> 
> Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>

I'm sorry, but no, ...

> --- a/xen/arch/arm/include/asm/p2m.h
> +++ b/xen/arch/arm/include/asm/p2m.h
> @@ -137,6 +137,8 @@ typedef enum {
>      p2m_max_real_type,  /* Types after this won't be store in the p2m */
>  } p2m_type_t;
>  
> +#define p2m_mmio_direct p2m_mmio_direct_dev

... this isn't what I suggested. When Arm has three p2m_mmio_direct_*,
randomly aliasing one to p2m_mmio_direct is imo more likely to create
confusion than to help things. Imo you want to introduce ...

> --- a/xen/common/device-tree/dom0less-build.c
> +++ b/xen/common/device-tree/dom0less-build.c
> @@ -185,7 +185,7 @@ static int __init handle_passthrough_prop(struct 
> kernel_info *kinfo,
>                                 gaddr_to_gfn(gstart),
>                                 PFN_DOWN(size),
>                                 maddr_to_mfn(mstart),
> -                               p2m_mmio_direct_dev);
> +                               p2m_mmio_direct);

... a per-arch inline function which returns the type to use here.
The name of the function would want to properly reflect the purpose;
my limited DT knowledge may make arch_dt_passthrough_p2m_type() an
entirely wrong suggestion.

Jan