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

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