Re: [Xen-devel] [PATCH v2] x86: cap address bits CPUID output

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.05.16 at 15:21, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 09/05/16 14:15, Jan Beulich wrote:
>> Don't use more or report more to guests than we are capable of
>> handling.
>>
>> At once
>> - correct the involved extended CPUID level checks,
>> - simplify the code in hvm_cpuid() and mtrr_top_of_ram().
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, with perhaps one
> tweak
> 
>> --- a/xen/arch/x86/hvm/hvm.c
>> +++ b/xen/arch/x86/hvm/hvm.c
>> @@ -3504,19 +3504,19 @@ void hvm_cpuid(unsigned int input, unsig
>>          break;
>>  
>>      case 0x80000008:
>> +        *eax &= 0xff;
>>          count = d->arch.paging.gfn_bits + PAGE_SHIFT;
>> -        if ( (*eax & 0xff) > count )
>> -            *eax = (*eax & ~0xff) | count;
>> +        if ( *eax > count )
>> +            *eax = count;
>>  
>>          hvm_cpuid(1, NULL, NULL, NULL, &_edx);
>>          count = _edx & (cpufeat_mask(X86_FEATURE_PAE) |
>>                          cpufeat_mask(X86_FEATURE_PSE36)) ? 36 : 32;
>> -        if ( (*eax & 0xff) < count )
>> -            *eax = (*eax & ~0xff) | count;
>> +        if ( *eax < count )
>> +            *eax = count;
>>  
>>          hvm_cpuid(0x80000001, NULL, NULL, NULL, &_edx);
>> -        *eax = (*eax & ~0xffff00) | (_edx & cpufeat_mask(X86_FEATURE_LM)
>> -                                     ? 0x3000 : 0x2000);
>> +        *eax |= _edx & cpufeat_mask(X86_FEATURE_LM) ? vaddr_bits << 8 : 
>> 0x2000;

In that case I'd prefer

        *eax |= (_edx & cpufeat_mask(X86_FEATURE_LM) ? vaddr_bits : 32) << 8;

Jan



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