Re: [PATCH v3 2/8] x86/svm: silently drop writes to SYSCFG and related MSRs

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/09/2020 09:36, Jan Beulich wrote:
> On 01.09.2020 12:54, Roger Pau Monne wrote:
>> The SYSCFG, TOP_MEM1 and TOP_MEM2 MSRs are currently exposed to guests
>> and writes are silently discarded. Make this explicit in the SVM code
>> now, and just return default constant values when attempting to read
>> any of the MSRs, while continuing to silently drop writes.
>>
>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>
>> --- a/xen/arch/x86/hvm/svm/svm.c
>> +++ b/xen/arch/x86/hvm/svm/svm.c
>> @@ -1917,6 +1917,9 @@ static int svm_msr_read_intercept(unsigned int msr, 
>> uint64_t *msr_content)
>>              goto gpf;
>>          break;
>>  
>> +    case MSR_K8_SYSCFG:
>> +    case MSR_K8_TOP_MEM1:
>> +    case MSR_K8_TOP_MEM2:
>>      case MSR_K8_VM_CR:
>>          *msr_content = 0;
>>          break;
> Andrew, since you did suggest otherwise before, may I ask for an
> explicit statement of yours here, be it in ack/nak form or something
> less formal?

I'm not entirely convinced it is a safe thing to do, but lets see what
happens.

There is likely to be a bug tail from the flipping the default behaviour
at the end of the series, and this is certainly a simpler set of logic
than the alternative.

~Andrew