Re: [PATCH v4 8/8] x86/mm: adjust loop in arch_init_memory() to iterate over the PDX space

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.08.2025 11:52, Roger Pau Monne wrote:
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -275,7 +275,7 @@ static void __init assign_io_page(struct page_info *page)
>  
>  void __init arch_init_memory(void)
>  {
> -    unsigned long i, pfn, rstart_pfn, rend_pfn, iostart_pfn, ioend_pfn;
> +    unsigned long i, pfn, rstart_pfn, rend_pfn, iostart_pfn, ioend_pfn, pdx;
>  
>      /*
>       * Basic guest-accessible flags:
> @@ -328,9 +328,20 @@ void __init arch_init_memory(void)
>              destroy_xen_mappings((unsigned long)mfn_to_virt(iostart_pfn),
>                                   (unsigned long)mfn_to_virt(ioend_pfn));
>  
> -        /* Mark as I/O up to next RAM region. */
> -        for ( ; pfn < rstart_pfn; pfn++ )
> +        /*
> +         * Mark as I/O up to next RAM region.  Iterate over the PDX space to
> +         * skip holes which would always fail the mfn_valid() check.
> +         *
> +         * pfn_to_pdx() requires a valid (iow: RAM) PFN to convert to PDX,
> +         * hence provide pfn - 1, which is the tailing PFN from the last RAM
> +         * range, or pdx 0 if the input pfn is 0.
> +         */
> +        for ( pdx = pfn ? pfn_to_pdx(pfn - 1) + 1 : 0;
> +              pdx < pfn_to_pdx(rstart_pfn);
> +              pdx++ )
>          {
> +            pfn = pdx_to_pfn(pdx);
> +
>              if ( !mfn_valid(_mfn(pfn)) )
>                  continue;
>  

As much as I would have liked to ack this, I fear there's another caveat here:
At the top of the loop we check not only for RAM, but also for UNUSABLE. The
latter, like RAM, shouldn't be marked I/O, but we also can't use PFN <-> PDX
transformations on any such page.

Jan