Re: [PATCH v2] selftests: KVM: avoid failures due to reserved HyperTransport region

On 12/9/21 22:47, Sean Christopherson wrote:

...

• /*
  

•  * Otherwise it's at the top of the physical address
  

•  * space, possibly reduced due to SME by bits 11:6 of
  

•  * CPUID[0x8000001f].EBX.
  

•  */
  

• eax = 0x80000008;
  

• cpuid(&eax, &ebx, &ecx, &edx);
  

Should't this check 0x80000000.eax >= 0x80000008 first? Or do we just accept failure if family==0x17 and there's no 0x80000008? One paranoid option would be to use the pre-fam17 value, e.g.

     /* Before family 17h, the HyperTransport area is just below 1T. */
     ht_gfn = (1 << 28) - num_ht_pages;
     if (x86_family(eax) < 0x17)
             goto out;

     eax = 0x80000000;
     cpuid(&eax, &ebx, &ecx, &edx);
     max_ext_leaf = eax;

     /* Use the old, conservative value if MAXPHYADDR isn't enumerated. */
     if (max_ext_leaf < 0x80000008)
             goto out;

Yes, this works for me too. Though in practice I don't think any 64-bit machine ever existed without 0x80000008 (you need it to decide what's a canonical address and what isn't), so that would have to be a 32-bit fam17h machine.

Paolo

     /* comment */
     eax = 0x80000008;
     cpuid(&eax, &ebx, &ecx, &edx);
     max_pfn = (1ULL << ((eax & 255) - vm->page_shift)) - 1;
     if (max_ext_leaf >= 0x8000001f) {
             <adjust>
     }
     ht_gfn = max_pfn - num_ht_pages;

out: return min(max_gfn, ht_gfn - 1);

...

•         max_pfn = (1ULL << ((eax & 255) - vm->page_shift)) - 1;
  

LOL, "& 255", you just couldn't resist, huh? My version of Rami Code only goes up to 15.:-)