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

[Keir Fraser <keir.xen@xxxxxxxxx>]

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.

 -- 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.
>> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel