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

[Sean Christopherson]

On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> From ef78673f78e3f2eedc498c1fbf9271146caa83cb Mon Sep 17 00:00:00 2001
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
> Date: Thu, 15 Apr 2021 15:57:02 +0000
> Subject: [PATCH 2/3] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall
> 
> This hypercall is used by the SEV guest to notify a change in the page
> encryption status to the hypervisor. The hypercall should be invoked
> only when the encryption attribute is changed from encrypted -> decrypted
> and vice versa. By default all guest pages are considered encrypted.
> 
> The hypercall exits to userspace to manage the guest shared regions and
> integrate with the userspace VMM's migration code.

...

> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index fd4a84911355..2bc353d1f356 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6766,3 +6766,14 @@ they will get passed on to user space. So user space still has to have
>  an implementation for these despite the in kernel acceleration.
>  
>  This capability is always enabled.
> +
> +8.32 KVM_CAP_EXIT_HYPERCALL
> +---------------------------
> +
> +:Capability: KVM_CAP_EXIT_HYPERCALL
> +:Architectures: x86
> +:Type: vm
> +
> +This capability, if enabled, will cause KVM to exit to userspace
> +with KVM_EXIT_HYPERCALL exit reason to process some hypercalls.
> +Right now, the only such hypercall is KVM_HC_PAGE_ENC_STATUS.
> diff --git a/Documentation/virt/kvm/cpuid.rst b/Documentation/virt/kvm/cpuid.rst
> index cf62162d4be2..c9378d163b5a 100644
> --- a/Documentation/virt/kvm/cpuid.rst
> +++ b/Documentation/virt/kvm/cpuid.rst
> @@ -96,6 +96,11 @@ KVM_FEATURE_MSI_EXT_DEST_ID        15          guest checks this feature bit
>                                                 before using extended destination
>                                                 ID bits in MSI address bits 11-5.
>  
> +KVM_FEATURE_HC_PAGE_ENC_STATUS     16          guest checks this feature bit before
> +                                               using the page encryption state
> +                                               hypercall to notify the page state
> +                                               change

...

>  int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  {
>  	unsigned long nr, a0, a1, a2, a3, ret;
> @@ -8334,6 +8346,28 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  		kvm_sched_yield(vcpu, a0);
>  		ret = 0;
>  		break;
> +	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))

> +			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.

> +			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
>