Re: [Xen-devel] [PATCH v6.5 19/26] x86/hvm: Permit guests direct access to MSR_{SPEC_CTRL, PRED_CMD}

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/01/18 09:52, Jan Beulich wrote:
>
>> --- a/xen/arch/x86/msr.c
>> +++ b/xen/arch/x86/msr.c
>> @@ -132,7 +132,8 @@ int guest_rdmsr(const struct vcpu *v, uint32_t msr, 
>> uint64_t *val)
>>      case MSR_SPEC_CTRL:
>>          if ( !cp->feat.ibrsb )
>>              goto gp_fault;
>> -        *val = vp->spec_ctrl.guest;
>> +        *val = (vp->spec_ctrl.direct_access
>> +                ? vp->spec_ctrl.host : vp->spec_ctrl.guest);
>>          break;
> To recap, I had asked whether this is valid ahead of later changes,
> which you replied to saying this won't have any "by not permitting
> the guest any access until patch 25". In which case at the very
> least the patch title is misleading. Yet I don't even agree with what
> you say - patch 25 only fiddles with CPUID bits. Did you perhaps
> mean to say "By not permitting a well behaved guest any access
> until patch 25," as one trying to access the MSRs without consulting
> the CPUID bits would be able to starting with the patch here aiui?

The guest access bit being clear in cpufeatureset.h means that the
maximum featureset calculations for guests will guarantee that
cp->feat.ibrsb is currently false.

~Andrew

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