Re: [PATCH 0/3] KVM: x86: guest interface for SEV live migration

[Paolo Bonzini]

On 20/04/21 19:31, Sean Christopherson wrote:
> > +	case KVM_HC_PAGE_ENC_STATUS: {
> > +		u64 gpa = a0, npages = a1, enc = a2;
> > +
> > +		ret = -KVM_ENOSYS;
> > +		if (!vcpu->kvm->arch.hypercall_exit_enabled)
>
> I don't follow, why does the hypercall need to be gated by a capability?  What
> would break if this were changed to?
>
> 		if (!guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS))

The problem is that it's valid to take KVM_GET_SUPPORTED_CPUID and send 
it unmodified to KVM_SET_CPUID2.  For this reason, features that are 
conditional on other ioctls, or that require some kind of userspace 
support, must not be in KVM_GET_SUPPORTED_CPUID.  For example:

- TSC_DEADLINE because it is only implemented after KVM_CREATE_IRQCHIP 
(or after KVM_ENABLE_CAP of KVM_CAP_IRQCHIP_SPLIT)

- MONITOR only makes sense if userspace enables KVM_CAP_X86_DISABLE_EXITS

X2APIC is reported even though it shouldn't be.  Too late to fix that, I 
think.

In this particular case, if userspace sets the bit in CPUID2 but doesn't 
handle KVM_EXIT_HYPERCALL, the guest will probably trigger some kind of 
assertion failure as soon as it invokes the HC_PAGE_ENC_STATUS hypercall.

(I should document that, Jim asked for documentation around 
KVM_GET_SUPPORTED_CPUID and KVM_GET_MSR_INDEX_LIST many times).

Paolo

> > +			break;
> > +
> > +		if (!PAGE_ALIGNED(gpa) || !npages ||
> > +		    gpa_to_gfn(gpa) + npages <= gpa_to_gfn(gpa)) {
> > +			ret = -EINVAL;
> > +			break;
> > +		}
> > +
> > +		vcpu->run->exit_reason        = KVM_EXIT_HYPERCALL;
> > +		vcpu->run->hypercall.nr       = KVM_HC_PAGE_ENC_STATUS;
> > +		vcpu->run->hypercall.args[0]  = gpa;
> > +		vcpu->run->hypercall.args[1]  = npages;
> > +		vcpu->run->hypercall.args[2]  = enc;
> > +		vcpu->run->hypercall.longmode = op_64_bit;
> > +		vcpu->arch.complete_userspace_io = complete_hypercall_exit;
> > +		return 0;
> > +	}
> >   	default:
> >   		ret = -KVM_ENOSYS;
> >   		break;
>
> ...
>
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 590cc811c99a..d696a9f13e33 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3258,6 +3258,14 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >   		vcpu->arch.msr_kvm_poll_control = data;
> >   		break;
> >   
> > +	case MSR_KVM_MIGRATION_CONTROL:
> > +		if (data & ~KVM_PAGE_ENC_STATUS_UPTODATE)
> > +			return 1;
> > +
> > +		if (data && !guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS))
>
> Why let the guest write '0'?  Letting the guest do WRMSR but not RDMSR is
> bizarre.

Because it was the simplest way to write the code, but returning 0 
unconditionally from RDMSR is actually simpler.

Paolo

> > +			return 1;
> > +		break;
> > +
> >   	case MSR_IA32_MCG_CTL:
> >   	case MSR_IA32_MCG_STATUS:
> >   	case MSR_IA32_MC0_CTL ... MSR_IA32_MCx_CTL(KVM_MAX_MCE_BANKS) - 1:
> > @@ -3549,6 +3557,12 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >   		if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF))
> >   			return 1;
> >   
> > +		msr_info->data = 0;
> > +		break;
> > +	case MSR_KVM_MIGRATION_CONTROL:
> > +		if (!guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS))
> > +			return 1;
> > +
> >   		msr_info->data = 0;
> >   		break;
> >   	case MSR_KVM_STEAL_TIME:
> > --
> > 2.26.2