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Re: [Xen-devel] Can't map the page referenced by HVM-DomU CR3 in Dom0

To: Keir Fraser <keir.xen@xxxxxxxxx>
Subject: Re: [Xen-devel] Can't map the page referenced by HVM-DomU CR3 in Dom0
From: david <david_n@xxxxxx>
Date: Tue, 19 Apr 2011 23:58:04 +0200
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On 04/19/2011 06:26 PM, Keir Fraser wrote:
On 19/04/2011 16:44, "david"<david_n@xxxxxx>  wrote:

On 04/18/2011 11:34 AM, Tim Deegan wrote:
At 14:45 +0100 on 15 Apr (1302878734), david wrote:
I'm trying to access the page containing the paging information for a
DomU from Dom0.

I'm doing that by translating the address contained in the DomU CR3
register with xc_translate_foreign_address (libxc) and try to map the
returned frame number with xc_map_foreign_range.

The problem is, that the return value from xc_translate_foreign_address
is 0 (guest cr3 is 0x002f3000 in my case), which indicates an error
(corresponding to the code comments). After some debugging I have
discovered, that pte becomes 0 when level=2 and therefore the function
returns 0 on line 79:

How often does this happen?  On every attempt or only from time to time?
Have you checked (say, from inside the guest) that the level-2 PTE isn't
actually zero?

hi,

it happens for every cr3 value. I made some quick and dirty code, which
reads 10 different cr3 values and tries to map the corresponding page:

xc_translate_foreign_address() will convert a guest virtual address into a
guest physical address. It's not working out for you because guest cr3
values are already guest physical addresses. Thus the virtual-to-physical
conversion you subject the values to is meaningless.

ahhh, .. I see. I thought the translation is between guest physical and real physical address. The way to calculate the correct physical frame number is cr3 >> PAGE_SHIFT corresponding to xen_cr3_to_pfn(cr3) in public/arch-x86/xen-x86_64.h, right?

thanks for your help,

greets
david



  -- Keir

----------------------------------------------------------------------
...
...
int crfinder = 1;

if(crfinder == 1){
          int m;
          unsigned long cr3s[10] = {0};
          unsigned long mfn = 0;
          vcpu_guest_context_any_t *ctxt =
malloc(sizeof(vcpu_guest_context_any_t));
          unsigned long cr3 = ctxt->c.ctrlreg[3];

          while(1 == 1){

                  xc_vcpu_getcontext(xcinterface, domain,
dominfo.max_vcpu_id, ctxt);
                  cr3 = ctxt->c.ctrlreg[3];

                  for(m = 0; m<  10; m++){

                          //already stored?
                          if(cr3s[m] == cr3){

                                  break;
                          //checked all stored cr3 values?
                          }else if (cr3s[m] != 0){

                                  continue;
                          //obviously new one found
                          }else{
                                  cr3s[m] = cr3;
                                  printf("new cr3 found %08x, stored in
%d\n", cr3, m);

                                  mfn =
xc_translate_foreign_address(xcinterface, domain, 0, cr3s[m]);
                                  printf("calculated mfn %08d for address
%08x\n", mfn, cr3s[m]);
                                  break;
                          }
                  }

                  if(m == 10)
                         return 0;
          }
}
----------------------------------------------------------------------

the corresponding output is:

new cr3 found 002f3000, stored in 0
calculated mfn 00000000 for address 002f3000
new cr3 found 06ac01a0, stored in 1
calculated mfn 00000000 for address 06ac01a0
new cr3 found 06ac0040, stored in 2
calculated mfn 00000000 for address 06ac0040
new cr3 found 06ac00a0, stored in 3
calculated mfn 00000000 for address 06ac00a0
new cr3 found 06ac01e0, stored in 4
calculated mfn 00000000 for address 06ac01e0
new cr3 found 06ac0320, stored in 5
calculated mfn 00000000 for address 06ac0320
new cr3 found 06ac02a0, stored in 6
calculated mfn 00000000 for address 06ac02a0
new cr3 found 06ac01c0, stored in 7
calculated mfn 00000000 for address 06ac01c0
new cr3 found 06ac0200, stored in 8
calculated mfn 00000000 for address 06ac0200
new cr3 found 06ac0060, stored in 9
calculated mfn 00000000 for address 06ac0060

so, every try to translate a cr3 address to a frame number (I don't know
what's the correct wording for frame numbers in hvm domains, .. mfn?)
ends in 0. Maybe it's a failure in my code? I can't find it currently :)
... I'm trying now, to read the cr3 values inside the domain, to check
if the values are the same.

greets,
david



Cheers,

Tim.


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