Re: [PATCH v2 1/8] x86/pdx: simplify calculation of domain struct allocation boundary

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.06.2025 13:11, Roger Pau Monne wrote:
> When not using CONFIG_BIGMEM there are some restrictions in the address
> width for allocations of the domain structure, as it's PDX truncated to 32
> bits it's stashed into page_info structure for domain allocated pages.
> 
> The current logic to calculate this limit is based on the internals of the
> PDX compression used, which is not strictly required.  Instead simplify the
> logic to rely on the existing PDX to PFN conversion helpers used elsewhere.
> 
> This has the added benefit of allowing alternative PDX compression
> algorithms to be implemented without requiring to change the calculation of
> the domain structure allocation boundary.
> 
> As a side effect introduce pdx_to_paddr() conversion macro and use it.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> @@ -498,14 +474,20 @@ struct domain *alloc_domain_struct(void)
>       * On systems with CONFIG_BIGMEM there's no packing, and so there's no
>       * such restriction.
>       */
> -#if defined(CONFIG_BIGMEM) || !defined(CONFIG_PDX_COMPRESSION)
> -    const unsigned int bits = IS_ENABLED(CONFIG_BIGMEM) ? 0 :
> -                                                          32 + PAGE_SHIFT;
> +#if defined(CONFIG_BIGMEM)
> +    const unsigned int bits = 0;
>  #else
> -    static unsigned int __read_mostly bits;
> +    static unsigned int __ro_after_init bits;
>  
>      if ( unlikely(!bits) )
> -         bits = _domain_struct_bits();
> +         /*
> +          * Get the width for the next pfn, and unconditionally subtract one
> +          * from it to ensure the used width will not allocate past the PDX
> +          * field limit.
> +          */
> +         bits = flsl(pdx_to_paddr(1UL << (sizeof_field(struct page_info,
> +                                                       v.inuse._domain) * 
> 8)))

You didn't like the slightly shorter sizeof(frame_table->v.inuse._domain) then?

Jan