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

[Konrad Rzeszutek Wilk]

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.