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Re: [Xen-users] Re: [Xen-devel] Xen - Guest memory allocation

  • To: "dinesh chandrasekaran" <dinesh_chan8@xxxxxxxxxxx>
  • From: "Derek Murray" <Derek.Murray@xxxxxxxxxxxx>
  • Date: Fri, 20 Jun 2008 09:56:30 +0100
  • Cc: xen developers community <xen-devel@xxxxxxxxxxxxxxxxxxx>, xen users community <xen-users@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Fri, 20 Jun 2008 01:56:53 -0700
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Hi Dinesh,

On Fri, Jun 20, 2008 at 3:34 AM, dinesh chandrasekaran
<dinesh_chan8@xxxxxxxxxxx> wrote:
> The domain builder inside dom0 calls populate_physmap(),
> allocates pages for domUs,
> and computes the following START_INFO_PAGE attributes
>                   pt_base, mfn_list and nr_pt_frames.
> But the source says, all the above attributes are VIRTUAL ADDRESSES
> and on the contrary the book, 'The defenitive guide for XEN hypervisor' says
> they are pseudo-physical addresses.
> I know these two are different. But I'm still not clear,
> Could you help me better understand the difference?

The comment in the xen.h that refers to virtual addresses means that
these pt_base and mfn_list contain pointers (in the virtual address
space) to the bootstrap page table and list of MFNs (machine frame
numbers, corresponding to the real (host) physical addresses). When a
paravirtualised domain is created, it is already in protected mode
with paging enabled, and the domain builder creates a bootstrap page
table for it to use (so that it can address the kernel, modules and
other virtualisation-related data structures).

I don't have a copy of that book, but I assume what it means is that
the array mfn_list is *indexed* by pseudo-physical frame numbers (i.e.
one for each frame of physical memory allocated to the guest,
contiguously numbered and starting from zero).



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