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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Aug 05, 2025 at 02:38:38PM +0200, Jan Beulich wrote:
> 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.

Right you are.  I'm not sure however why we do this - won't we want
the mappings of UNUSABLE regions also be removed from the Xen
page-tables? (but not marked as IO)

I could do something like:

        /* Mark as I/O up to next RAM or UNUSABLE region. */
        if ( (!pfn || pdx_is_region_compressible(pfn_to_paddr(pfn - 1), 1)) &&
             pdx_is_region_compressible(pfn_to_paddr(rstart_pfn), 1) )
        {
            /*
             * 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;

                assign_io_page(mfn_to_page(_mfn(pfn)));
            }
        }
        else
        {
            /* Slow path, iterate over the PFN space. */
            for ( ; pfn < rstart_pfn; pfn++ )
            {
                if ( !mfn_valid(_mfn(pfn)) )
                    continue;

                assign_io_page(mfn_to_page(_mfn(pfn)));
            }
        }

But I find it a bit ugly - I might send v5 without this final patch
while I see if I can find a better alternative.

Thanks, Roger.