RE: [Xen-devel] Comment & code inconsistency?

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: 陈诚
>Sent: 2007年11月18日 22:20
>
>Hi all,
>    in arch_init_memory() we have the following code:
>
>    ...
>
>    /* Any areas not specified as RAM by the e820 map are 
>considered I/O. */
>    for ( i = 0, pfn = 0; pfn < max_page; i++ )
>    {
>        while ( (i < e820.nr_map) && (e820.map[i].type != E820_RAM) )
>            i++;
>
>        if ( i >= e820.nr_map )
>        {
>            /* No more RAM regions: mark as I/O right to end 
>of memory map. */
>            rstart_pfn = rend_pfn = max_page;
>        }
>        else
>        {
>            /* Mark as I/O just up as far as next RAM region. */
>            rstart_pfn = min_t(unsigned long, max_page,
>                               PFN_UP(e820.map[i].addr));
>            rend_pfn   = max_t(unsigned long, rstart_pfn,
>                               PFN_DOWN(e820.map[i].addr + 
>e820.map[i].size));
>        }
>
>        /* Mark as I/O up to next RAM region. */
>        for ( ; pfn < rstart_pfn; pfn++ )
>        {
>            BUG_ON(!mfn_valid(pfn));
>            share_xen_page_with_guest(
>                mfn_to_page(pfn), dom_io, XENSHARE_writable);
>        }
>
>        /* Skip the RAM region. */
>        pfn = rend_pfn;
>    }
>
>    ...
>
>    it seems that the while loop skips all the areas that are not
>specified as RAM and the areas which are specified as RAM are
>considered I/O, which contradicts with the comment, or do I
>misunderstand this piece of code?
>

It matches. Above code just adds frames from end of last RAM entry 
to start of next RAM entry. Not all non-RAM ranges are described in 
E820 table, which is why you need instead anchor RAM entries and 
get the reverse.

Thanks,
Kevin

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