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[Xen-devel] ARM - why does setup_frametable_size() round frametable_size to 32MB ?

  • To: "xen-devel@xxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxx>
  • From: "Chris (Christopher) Brand" <chris.brand@xxxxxxxxxxxx>
  • Date: Fri, 17 Jul 2015 21:19:04 +0000
  • Accept-language: en-US
  • Delivery-date: Fri, 17 Jul 2015 21:19:31 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>
  • Thread-index: AdDA1OBA8EgxKe5+T+eAajAmYHft2A==
  • Thread-topic: ARM - why does setup_frametable_size() round frametable_size to 32MB ?


I'm working on a platform with a mere 2GB of RAM, and trying to
trim the Xen footprint down as much as possible. I've found two
places where Xen uses more memory than it seems it needs to,
one of which is the frametable. On a 2GB system, frametable_size
is initially calculated as 16MB, but is then rounded up to 32MB.
can somebody tell me why this is done, and therefore whether
it can be avoided ? I assume it's because the code then calls
create_32mb_mappings(), in which case I guess my question
is what's special about 32MB ?

    unsigned long frametable_size = nr_pdxs * sizeof(struct page_info);
    /* Round up to 32M boundary */
    frametable_size = (frametable_size + 0x1ffffff) & ~0x1ffffff;



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