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Re: [Xen-devel] [Paging] 1GB pages in PV DomU

  • To: "FranÃois-FrÃdÃric Ozog" <ff@xxxxxxxx>
  • From: "Jan Beulich" <JBeulich@xxxxxxxx>
  • Date: Wed, 09 Jan 2013 15:35:54 +0000
  • Cc: xen-devel@xxxxxxxxxxxxx
  • Delivery-date: Wed, 09 Jan 2013 15:36:48 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>

>>> On 09.01.13 at 16:25, FranÃois-FrÃdÃric Ozog<ff@xxxxxxxx> wrote:
> I need to map large portions (say 64GB) of mini-os virtual address space of
> a PV DomU to 1GB machine pages on top of xen-unstable (processor has all
> required capabilities and resources).
> When creating a 10GB domain, I can find sets of machine 4KB pages in
> start_info.mfn_list that can be grouped to build 1GB page but they appear in
> "descending" order. So wen I build my domain page table
> (arch/x86/mm.c:build_pagetable), I make sure I use 512 adjacent pages that
> can be 1GB aligned and try to mark the corresponding L3 table.

For one, 512 pages sum up to only 2Mb, not 1Gb.

Next, without enhancing the hypervisor to support this, the
biggest mappings you can create in a PV guest are 2Mb (and
all you should need is enable the support on the Xen command
line and in the guest config file(s) (for those where you need
it). Then the domain builder should be populating the physical
address space with contiguous 2Mb chunks.


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