Xen devel - Feb 2007 - How to map machine address and pseudo-physical address?

Hello evrybody,

  I need to parse data structures supplied by the BIOS in machine memory.
I''d like to know how I can do it in dom0? 

  I though about mapping part of memory machine in dom0 but I have the
following problem. I take the example of code taken from Linux in
arch/i386/kernel/summit.c to get the pointer to the EBDA at physical @
0x40E:

...
    unsigned long           ptr;

    /* The pointer to the EBDA is stored in the word @ phys 
       0x40E(40:0E)*/
    ptr = *(unsigned short *)phys_to_virt(0x40Eul);
    ptr = (unsigned long)phys_to_virt(ptr << 4);
...

On a linux kernel, ptr==0xffff810000099000
If I run the same code inside dom0, I get ptr==0xffff880000000000

  It''s normal because under dom0 the 0X40E corresponds to the pseudo
physical address and not to the machine address. So the idea (maybe
it''s
not the good one, it''s a try :) is to map the machine address 0x40E to
pseudo physical address 0x40E and then, I should read the correct
pointer value from dom0. Thus in Xen I add a new ''case'' in the
function arch/x86/plateform_hypercall.c:do_platform_op() that is:

....
    case XENPF_set_ebda_ptr:
    {
       unsigned long ebda_ptr = op->u.set_ebda_ptr.val;
       unsigned long ptr;
        
        /* the ptr value is used to see if the mapping is working */
        /* The pointer to the EBDA is stored in the word @ maddr
           0x40E(40:0E) */
        ptr = *(unsigned short*)maddr_to_virt(ebda_ptr); 
        ptr = (unsigned long)maddr_to_virt(ptr << 4);
        printk(KERN_INFO "TESTGuill: ptr = 0x%lx \n", ptr);
        set_gpfn_from_mfn(ebda_ptr, ebda_ptr);
    }       
    break;
....

  I also add needed structure. Now, before reading ptr in dom0, I make an
hypercall to map machine address 0x40E (value passed in
op->u.set_ebda_ptr.val) and I can see in the boot messages the
following:

(XEN) TESTGuill: ptr = 0xffff830000099000
TESTGuill: ptr = 0xffff880000000000

  I expected to see the same value under Xen and under dom0. The value
0xffff830000099000 read under Xen seems OK because it corresponds to 1:1
direct mapping of all physical memory so 0x99000 is the right value.

 So the question is: Is machine memory mapped in dom0? if not, can I map
parts of it?


Regards,
Guillaume 

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>  So the question is: Is machine memory mapped in dom0? if not, can  
> I map
> parts of it?
If I understand your question correctly, it sounds like you will want  
to use the xc_map_foreign_range function from libxc.  That function  
will take an mfn, so you will need to convert the pseudo-physical  
address and convert it to a machine address first.  There are mapping  
tables in xen to help you do this, but I have only used them for  
viewing memory from domU, not from the bios.

You might find some useful code in XenAccess as much of these  
techniques are implemented in there.  You can download the code from  
xenaccess.sf.net.

Cheers,
bryan


-
Bryan D. Payne
Graduate Student, Computer Science
Georgia Tech Information Security Center
http://www.bryanpayne.org



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On Wed, 28 Feb 2007 17:30:58 -0500
"Bryan D. Payne" <bryan@thepaynes.cc> wrote:
> >  So the question is: Is machine memory mapped in dom0? if not, can  
> > I map
> > parts of it?
> 
> If I understand your question correctly, it sounds like you will want  
> to use the xc_map_foreign_range function from libxc. 
From my understanding (please correct me if I''m wrong), libxc is (more
or less) for Xen+dom0 what glibc is for Linux. It''s a set of functions
that allows interactions between Xen/Dom0 and other domains. 

Me, I need to parse machine memory (that contains information provided
by BIOS) from Dom0 during Dom0''s boot.

As I''m not 100% sure I will have a closer look to your proposition but
I
think that it doesn''t fit to my need.

Thanks for your answer Bryan,
Regards,
Guillaume

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> Me, I need to parse machine memory (that contains information provided
> by BIOS) from Dom0 during Dom0''s boot.
Libxc is just a convenient way to get at this functionality.  If you  
need to map memory while booting dom0, then you can still do this by  
placing code in the dom0 kernel.  You would just need to call the  
same hypercall that libxc uses to map foreign memory ranges.  If you  
need to do it while booting xen, then you can do similar things  
inside the hypervisor.

-bryan


-
Bryan D. Payne
Graduate Student, Computer Science
Georgia Tech Information Security Center
http://www.bryanpayne.org



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