Re: [PATCH v2 2/2] x86/mm: drop unmapping from marking-as-I/O in arch_init_memory()

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Aug 11, 2025 at 12:50:23PM +0200, Jan Beulich wrote:
> The unmapping part would have wanted to cover UNUSABLE regions as well,
> and it would now have been necessary for space outside the low 16Mb
> (wherever Xen is placed). However, with everything up to the next 2Mb
> boundary now properly backed by RAM, we don't need to unmap anything
> anymore: Space up to __2M_rwdata_end[] is properly reserved, whereas
> space past that mark (up to the next 2Mb boundary) is ordinary RAM.

Oh, I see, so this was done to unmap trailing space when the Xen image
region is mapped using 2M pages.

> While there, limit the scopes of involved variables.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

> ---
> v2: Drop unmapping code altogether.
> 
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -275,8 +275,6 @@ static void __init assign_io_page(struct
>  
>  void __init arch_init_memory(void)
>  {
> -    unsigned long i, pfn, rstart_pfn, rend_pfn, iostart_pfn, ioend_pfn;
> -
>      /*
>       * Basic guest-accessible flags:
>       *   PRESENT, R/W, USER, A/D, AVAIL[0,1,2], AVAIL_HIGH, NX (if available).
> @@ -292,12 +290,17 @@ void __init arch_init_memory(void)
>       * case the low 1MB.
>       */
>      BUG_ON(pvh_boot && trampoline_phys != 0x1000);
> -    for ( i = 0; i < 0x100; i++ )
> +    for ( unsigned int i = 0; i < MB(1) >> PAGE_SHIFT; i++ )

I would use PFN_DOWN() rather than the shift, but that's just my
preference.

Thanks, Roger.