Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

[KarimAllah Ahmed]

On 02/01/2018 03:19 PM, Konrad Rzeszutek Wilk wrote:
> .snip..
> > > +/* Is SPEC_CTRL intercepted for the currently running vCPU? */
> > > +static bool spec_ctrl_intercepted(struct kvm_vcpu *vcpu)
> > > +{
> > > +	unsigned long *msr_bitmap;
> > > +	int f = sizeof(unsigned long);
> > > +
> > > +	if (!cpu_has_vmx_msr_bitmap())
> > > +		return true;
> > > +
> > > +	msr_bitmap = is_guest_mode(vcpu) ?
> > > +			to_vmx(vcpu)->nested.vmcs02.msr_bitmap :
> > > +			to_vmx(vcpu)->vmcs01.msr_bitmap;
> > > +
> > > +	return !!test_bit(MSR_IA32_SPEC_CTRL, msr_bitmap + 0x800 / f);
> > > +}
> > > +
> ..snip..
> > > @@ -3359,6 +3393,34 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > >   	case MSR_IA32_TSC:
> > >   		kvm_write_tsc(vcpu, msr_info);
> > >   		break;
> > > +	case MSR_IA32_SPEC_CTRL:
> > > +		if (!msr_info->host_initiated &&
> > > +		    !guest_cpuid_has(vcpu, X86_FEATURE_IBRS) &&
> > > +		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
> > > +			return 1;
> > > +
> > > +		vmx->spec_ctrl_used = true;
> > > +
> > > +		/* The STIBP bit doesn't fault even if it's not advertised */
> > > +		if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP))
> > > +			return 1;
> > > +
> > > +		vmx->spec_ctrl = data;
> > > +
> > > +		/*
> > > +		 * When it's written (to non-zero) for the first time, pass
> > > +		 * it through. This means we don't have to take the perf
> >
> > .. But only if it is a nested guest (as you have && is_guest_mode).
> >
> > Do you want to update the comment a bit?
> >
> > > +		 * hit of saving it on vmexit for the common case of guests
> > > +		 * that don't use it.
> > > +		 */
> > > +		if (cpu_has_vmx_msr_bitmap() && data &&
> > > +		    spec_ctrl_intercepted(vcpu) &&
> > > +		    is_guest_mode(vcpu))
> >                      ^^^^^^^^^^^^^^^^^^ <=== here
>
> Would it be perhaps also good to mention the complexity of how
> we ought to be handling L1 and L2 guests in the commit?
>
> We are all stressed and I am sure some of us haven't gotten much
> sleep - but it can help in say three months when some unluckly new
> soul is trying to understand this and gets utterly confused.

Yup, I will go through the patches and add as much details as possible.

And yes, the is_guest_mode(vcpu) here is inverted :D I blame the late
night :)

> > > +			vmx_disable_intercept_for_msr(
> > > +					vmx->vmcs01.msr_bitmap,
> > > +					MSR_IA32_SPEC_CTRL,
> > > +					MSR_TYPE_RW);
> > > +		break;
> > >   	case MSR_IA32_PRED_CMD:
> > >   		if (!msr_info->host_initiated &&
> > >   		    !guest_cpuid_has(vcpu, X86_FEATURE_IBPB) &&
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