Re: [PATCH v2 for-4.15] x86/msr: introduce an option for HVM relaxed rdmsr behavior

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.03.2021 11:56, Jan Beulich wrote:
> On 04.03.2021 15:47, Roger Pau Monne wrote:
>> --- a/xen/arch/x86/hvm/svm/svm.c
>> +++ b/xen/arch/x86/hvm/svm/svm.c
>> @@ -1795,6 +1795,7 @@ static int svm_msr_read_intercept(unsigned int msr, 
>> uint64_t *msr_content)
>>      const struct domain *d = v->domain;
>>      struct vmcb_struct *vmcb = v->arch.hvm.svm.vmcb;
>>      const struct nestedsvm *nsvm = &vcpu_nestedsvm(v);
>> +    uint64_t tmp;
>>  
>>      switch ( msr )
>>      {
>> @@ -1965,6 +1966,11 @@ static int svm_msr_read_intercept(unsigned int msr, 
>> uint64_t *msr_content)
>>          break;
>>  
>>      default:
>> +        if ( d->arch.hvm.rdmsr_relaxed && !rdmsr_safe(msr, tmp) )
>> +        {
>> +            *msr_content = 0;
>> +            break;
>> +        }
> 
> You don't really need "tmp" here, do you? You could as well read
> into *msr_content, as you're zapping the value afterwards anyway.

Actually, while perhaps indeed not strictly needed, it allows the
compiler to produce better code, as it'll be able to recognize
the value doesn't need writing to memory on any path. I guess I'll
change the logic in my related patch along these lines then.

Jan