Xen devel - Oct 2006 - Memory management, mapping, paging questions...

First hello everybody,

Currently I play with Xen (2.0.7), especially I want to know more about
the memory management. What I understood is that at start-of-day, in the
start_info structure, we have the following information:

nr_pages = Number of machine pages provided for the virtual machine. Represents
           the amount of memory specified in the configuration file (nr_pages *
           4096).
pt_base  = The virtual address of the page directory.
nr_pt_frames = Number of page tables created for the VM by Xen
mfn_list = List of machine adresses.

So my questions are the following :

* pt_base is the virtual adress of the pd. But what means virtual here ? in
which adress space does this adress has the right meaning ?
* Does Xen create the page tables (I do not include the pd in page tables, I
think it is more understable like this, i.e. pd != pt) ? Where are these pt?
their address ?
* mfn_list : is this a list, starting at adress mfn_list, that contains all the
machine adresses of all machine pages allocated for our VM ? i.e. the following
code print all the machine page adresses :
       for (i = 0; i < nr_pages; i++)
              printf("page %d at %x\n", i, ((unsigned long
*)mfn_list)[i]);
* If I want to map at identical (machine or physical adress == virtual address),
  how can I do ?

Thanks.

--
Nicolas CLERMONT
Xen player

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At Tue, 24 Oct 2006 17:11:37 +0200,
nicko.koinkoin@free.fr wrote:
> Currently I play with Xen (2.0.7), especially I want to know more about
> the memory management.
I hope I can answer your questions, but I''ve only used Xen 3/unstable
and I am also not one of the Xen experts (correct me if I''m wrong,
please!)
> So my questions are the following :
> 
> * pt_base is the virtual adress of the pd. But what means virtual
> here ? in which adress space does this adress has the right meaning
> ?
Xen has setup an address space which the guest domain starts in, so
pt_base is simply the virtual address of the page directory.
> * Does Xen create the page tables (I do not include the pd in page tables,
I
> think it is more understable like this, i.e. pd != pt) ? Where are these
pt?
> their address ?
You can always traverse the page directory page to find this out.
> * mfn_list : is this a list, starting at adress mfn_list, that contains all
the
> machine adresses of all machine pages allocated for our VM ? 
Yes, it stores the machine page frame numbers and it''s used to
translate physical addresses to machine addresses. I.e., something like

   unsigned long phys_to_machine(unsigned long phys)
   {
       unsigned long machine = mfn_list[phys >> 12];
       machine = (machine << 12) | (phys & (unsigned long)~4095);
       return machine;
   }

Mapping pages work basically as on real hardware except that you have
to translate physical addresses to machine addresses before entering
them into the page table.

// Simon

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