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

[david <david_n@gmx.at>]

On 04/19/2011 06:26 PM, Keir Fraser wrote:
> On 19/04/2011 16:44, "david"<david_n@gmx.at>  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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>
>
>