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


    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.
    >
    > > 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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