Re: [PATCH v3 2/4] KVM: x86: Add IBPB support

David Woodhouse
2018-01-30T14:36:18Z

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On Tue, 2018-01-30 at 08:22 -0600, Tom Lendacky wrote:
> > @@ -918,6 +919,9 @@ static void svm_vcpu_init_msrpm(u32 *msrpm)
> >
> >   set_msr_interception(msrpm, direct_access_msrs[i].index, 1, 1);
> >   }
> > +
> > + if (boot_cpu_has(X86_FEATURE_IBPB))
> > + set_msr_interception(msrpm, MSR_IA32_PRED_CMD, 1, 1);
>
> Not sure you really need the check here.  If the feature isn't available
> in the hardware, then it won't be advertised in the CPUID bits to the
> guest, so the guest shouldn't try to write to the msr.  If it does, it
> will #GP. So I would think it could be set all the time to not be
> intercepted, no?

The check for boot_cpu_has() is wrong and is fairly redundant as you
say. What we actually want is guest_cpu_has(). We *don't* want to pass
the MSR through for a recalcitrant guest to bash on, if we have elected
not to expose this feature to the guest.

On Intel right now it's *really* important that we don't allow it to be
touched, even if a write would succeed. So even boot_cpu_has() would
not be entirely meaningless there. :)

> > @@ -3330,6 +3331,14 @@ 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_PRED_CMD:
> > + if (!msr_info->host_initiated &&
> > +     !guest_cpuid_has(vcpu, X86_FEATURE_IBPB))
> > + return 1;
> > +
> > + if (data & PRED_CMD_IBPB)
> > + wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
> > + break;
>
> Should this also be in svm.c or as common code in x86.c?

See my response to [0/4]. I suggested that, but noted that it wasn't
entirely clear where we'd put the storage for SPEC_CTRL. We probably
*could* manage it for IBPB though.

> >
> >   case MSR_IA32_CR_PAT:
> >   if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {
> >   if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
> > @@ -9548,6 +9557,9 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
> >   goto free_msrs;
> >
> >   msr_bitmap = vmx->vmcs01.msr_bitmap;
> > +
> > + if (boot_cpu_has(X86_FEATURE_IBPB))
> > + vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PRED_CMD, MSR_TYPE_W);
>
> Same comment here as in svm.c, is the feature check necessary?

Again, yes but it should be guest_cpu_has() and we couldn't see how :)

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