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

From: Konrad Rzeszutek Wilk
Date: Mon Jun 04 2018 - 16:55:34 EST


On Mon, Jun 04, 2018 at 03:43:17PM -0500, Tom Lendacky wrote:
> 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.

<grins> I can imagine!
>
> >
> >
> > 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.

Excellent. Would you be OK giving it an Acked by or such?

Thanks.