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

[Paolo Bonzini <pbonzini@redhat.com>]

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
>>
>