Re: [Xen-devel] [PATCH 05/11] x86/emul: Unfold %cr4.de handling in x86emul_read_dr()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.06.18 at 18:03, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 06/06/18 15:20, Jan Beulich wrote:
>>>>> On 04.06.18 at 15:59, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> No functional change (as curr->arch.debugreg[5] is zero when DE is clear), 
> but
>>> this change simplifies the following patch.
>> A comment to this effect would be helpful ...
>>
>>> --- a/xen/arch/x86/x86_emulate.c
>>> +++ b/xen/arch/x86/x86_emulate.c
>>> @@ -101,23 +101,29 @@ int x86emul_read_dr(unsigned int reg, unsigned long 
> *val,
>>>      switch ( reg )
>>>      {
>>>      case 0 ... 3:
>>> -    case 6:
>>>          *val = curr->arch.debugreg[reg];
>>>          break;
>>>  
>>> +    case 4:
>>> +        if ( curr->arch.pv_vcpu.ctrlreg[4] & X86_CR4_DE )
>>> +            goto ud_fault;
>>> +
>>> +        /* Fallthrough */
>>> +    case 6:
>>> +        *val = curr->arch.debugreg[6];
>>> +        break;
>>> +
>>> +    case 5:
>>> +        if ( curr->arch.pv_vcpu.ctrlreg[4] & X86_CR4_DE )
>>> +            goto ud_fault;
>>> +
>>> +        /* Fallthrough */
>>>      case 7:
>>>          *val = (curr->arch.debugreg[7] |
>>>                  curr->arch.debugreg[5]);
>> ... somewhere above here. With that
>> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Even in light of the rename turning this into
> 
> *val = curr->arch.dr7 | curr->arch.pv_vcpu.dr7_emul;
> 
> ?

Indeed, the later adjustment makes a comment pretty pointless.

Jan



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