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

[david <david_n@xxxxxx>]

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