Re: [PATCH v1 2/3] x86/bugs: Add AMD's SPEC_CTRL MSR usage

[Tom Lendacky]

On 6/4/2018 3:20 PM, Konrad Rzeszutek Wilk wrote:
>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>> index 26110c202b19..950ec50f77c3 100644
>>> --- a/arch/x86/kvm/svm.c
>>> +++ b/arch/x86/kvm/svm.c
>>> @@ -4115,7 +4115,8 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>  		break;
>>>  	case MSR_IA32_SPEC_CTRL:
>>>  		if (!msr_info->host_initiated &&
>>> -		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS))
>>> +		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) &&
>>> +		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
>>
>> Shouldn't the IBRS/SSBD check be an "or" check?  I don't think it's
>> necessarily true that IBRS and SSBD have to both be set.  Maybe something
>> like:
>>
>> 	if (!msr_info->host_initiated &&
>> 	    !(guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) ||
>> 	      guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
>>
>> Does that make sense?
> 
> The '!' on each of the CPUID and '&&' make this the same. See:

Doh!  Yes, I don't know what I was thinking.  Just the end of a long
week I guess.

> 
> 
>  AMD_IBRS set	|  AMD_SSBD set	| !AMD_IBRS && !AMD_SSBD | !(AMD_IBRS || AMD_SSBD)
> 	0	|	0	| 1 && 1 -> return 1	 | !(0) -> 1 -> return 1
> 	1	|	0	| 0 && 1, continue	 | !(1 || 0) -> continue
> 	1	|	1	| 0 && 0, continue	 | !(1 || 1) -> continue
> 	0	|	1	| 1 && 0, continue	 | !(0 || 1) -> continue
> 
> Meaning we will return 1 if:
>  the host has not initiator it or,
>  the guest CPUID does not have AMD_IBRS flag or,
>  the guest CPUID does not have AMD SSBD flag
> 
> I am fine modifying it the way you had in mind, but in the past the logic
> was to use ! and &&, hence stuck to that.

No reason to change, it's fine the way you have it.

Thanks,
Tom

>>
>>>  			return 1;
>>>  
>>>  		msr_info->data = svm->spec_ctrl;
>>> @@ -4217,11 +4218,12 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>>>  		break;
>>>  	case MSR_IA32_SPEC_CTRL:
>>>  		if (!msr->host_initiated &&
>>> -		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS))
>>> +		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) &&
>>> +		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
>>
>> Same question as above.
>>
>> Thanks,
>> Tom
>>
>>>  			return 1;
>>>  
>>>  		/* The STIBP bit doesn't fault even if it's not advertised */
>>> -		if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP))
>>> +		if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD))
>>>  			return 1;
>>>  
>>>  		svm->spec_ctrl = data;
>>>