Re: [PATCH] KVM: SEV: Init target VMCBs in sev_migrate_from

[Sean Christopherson]

On Fri, Jun 17, 2022, Peter Gonda wrote:
> @@ -1681,6 +1683,10 @@ static void sev_migrate_from(struct kvm *dst_kvm, struct kvm *src_kvm)
>  
>  	list_cut_before(&dst->regions_list, &src->regions_list, &src->regions_list);
>  
> +	kvm_for_each_vcpu(i, vcpu, dst_kvm) {
> +		sev_init_vmcb(to_svm(vcpu));

Curly braces are unnecessary.

> +	}
> +
>  	/*
>  	 * If this VM has mirrors, "transfer" each mirror's refcount of the
>  	 * source to the destination (this KVM).  The caller holds a reference
> @@ -1739,6 +1745,8 @@ static int sev_es_migrate_from(struct kvm *dst, struct kvm *src)
>  		src_svm->vmcb->control.ghcb_gpa = INVALID_PAGE;
>  		src_svm->vmcb->control.vmsa_pa = INVALID_PAGE;
>  		src_vcpu->arch.guest_state_protected = false;
> +
> +		sev_es_init_vmcb(dst_svm);
>  	}
>  	to_kvm_svm(src)->sev_info.es_active = false;
>  	to_kvm_svm(dst)->sev_info.es_active = true;
> @@ -2914,6 +2922,12 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
>  				    count, in);
>  }
>  
> +void sev_init_vmcb(struct vcpu_svm *svm)
> +{
> +	svm->vmcb->control.nested_ctl |= SVM_NESTED_CTL_SEV_ENABLE;
> +	clr_exception_intercept(svm, UD_VECTOR);

I don't love separating SEV and SEV-ES VMCB initialization, especially since they're
both doing RMW operations and not straight writes.  E.g. migration ends up reversing
the order between the two relatively to init_vmcb().  That's just asking for a subtle
bug to be introduced that affects only due to the ordering difference.

What about using common top-level flows for SEV and SEV-ES so that the sequencing
between SEV and SEV-ES is more rigid?  The resulting sev_migrate_from() is a little
gross, but IMO it's worth having a fixed sequence, and the flip side to the ugliness
it that it documents some of the differences between SEV and SEV-ES migration.

---
 arch/x86/kvm/svm/sev.c | 75 +++++++++++++++++++++++++++---------------
 arch/x86/kvm/svm/svm.c | 11 ++-----
 arch/x86/kvm/svm/svm.h |  2 +-
 3 files changed, 52 insertions(+), 36 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 51fd985cf21d..9efb679d89d1 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1665,7 +1665,10 @@ static void sev_migrate_from(struct kvm *dst_kvm, struct kvm *src_kvm)
 {
 	struct kvm_sev_info *dst = &to_kvm_svm(dst_kvm)->sev_info;
 	struct kvm_sev_info *src = &to_kvm_svm(src_kvm)->sev_info;
+	struct kvm_vcpu *dst_vcpu, *src_vcpu;
+	struct vcpu_svm *dst_svm, *src_svm;
 	struct kvm_sev_info *mirror;
+	unsigned long i;

 	dst->active = true;
 	dst->asid = src->asid;
@@ -1704,27 +1707,22 @@ static void sev_migrate_from(struct kvm *dst_kvm, struct kvm *src_kvm)
 		list_del(&src->mirror_entry);
 		list_add_tail(&dst->mirror_entry, &owner_sev_info->mirror_vms);
 	}
-}

-static int sev_es_migrate_from(struct kvm *dst, struct kvm *src)
-{
-	unsigned long i;
-	struct kvm_vcpu *dst_vcpu, *src_vcpu;
-	struct vcpu_svm *dst_svm, *src_svm;
-
-	if (atomic_read(&src->online_vcpus) != atomic_read(&dst->online_vcpus))
-		return -EINVAL;
-
-	kvm_for_each_vcpu(i, src_vcpu, src) {
-		if (!src_vcpu->arch.guest_state_protected)
-			return -EINVAL;
-	}
-
-	kvm_for_each_vcpu(i, src_vcpu, src) {
-		src_svm = to_svm(src_vcpu);
-		dst_vcpu = kvm_get_vcpu(dst, i);
+	kvm_for_each_vcpu(i, dst_vcpu, dst_kvm) {
 		dst_svm = to_svm(dst_vcpu);

+		sev_init_vmcb(dst_svm);
+
+		if (!src->es_active)
+			continue;
+
+		/*
+		 * Note, the source is not required to have the same number of
+		 * vCPUs as the destination when migrating a vanilla SEV VM.
+		 */
+		src_vcpu = kvm_get_vcpu(dst_kvm, i);
+		src_svm = to_svm(src_vcpu);
+
 		/*
 		 * Transfer VMSA and GHCB state to the destination.  Nullify and
 		 * clear source fields as appropriate, the state now belongs to
@@ -1740,8 +1738,26 @@ static int sev_es_migrate_from(struct kvm *dst, struct kvm *src)
 		src_svm->vmcb->control.vmsa_pa = INVALID_PAGE;
 		src_vcpu->arch.guest_state_protected = false;
 	}
-	to_kvm_svm(src)->sev_info.es_active = false;
-	to_kvm_svm(dst)->sev_info.es_active = true;
+
+	dst->es_active = src->es_active;
+	src->es_active = false;
+}
+
+static int sev_check_source_vcpus(struct kvm *dst, struct kvm *src)
+{
+	struct kvm_vcpu *src_vcpu;
+	unsigned long i;
+
+	if (!sev_es_guest(src))
+		return 0;
+
+	if (atomic_read(&src->online_vcpus) != atomic_read(&dst->online_vcpus))
+		return -EINVAL;
+
+	kvm_for_each_vcpu(i, src_vcpu, src) {
+		if (!src_vcpu->arch.guest_state_protected)
+			return -EINVAL;
+	}

 	return 0;
 }
@@ -1789,11 +1805,9 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 	if (ret)
 		goto out_dst_vcpu;

-	if (sev_es_guest(source_kvm)) {
-		ret = sev_es_migrate_from(kvm, source_kvm);
-		if (ret)
-			goto out_source_vcpu;
-	}
+	ret = sev_check_source_vcpus(kvm, source_kvm);
+	if (ret)
+		goto out_source_vcpu;

 	sev_migrate_from(kvm, source_kvm);
 	kvm_vm_dead(source_kvm);
@@ -2914,7 +2928,7 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
 				    count, in);
 }

-void sev_es_init_vmcb(struct vcpu_svm *svm)
+static void sev_es_init_vmcb(struct vcpu_svm *svm)
 {
 	struct kvm_vcpu *vcpu = &svm->vcpu;

@@ -2967,6 +2981,15 @@ void sev_es_init_vmcb(struct vcpu_svm *svm)
 	}
 }

+void sev_init_vmcb(struct vcpu_svm *svm)
+{
+	svm->vmcb->control.nested_ctl |= SVM_NESTED_CTL_SEV_ENABLE;
+	clr_exception_intercept(svm, UD_VECTOR);
+
+	if (sev_es_guest(svm->vcpu.kvm))
+		sev_es_init_vmcb(svm);
+}
+
 void sev_es_vcpu_reset(struct vcpu_svm *svm)
 {
 	/*
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index c6cca0ce127b..a6bb67738005 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1259,15 +1259,8 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
 		svm->vmcb->control.int_ctl |= V_GIF_ENABLE_MASK;
 	}

-	if (sev_guest(vcpu->kvm)) {
-		svm->vmcb->control.nested_ctl |= SVM_NESTED_CTL_SEV_ENABLE;
-		clr_exception_intercept(svm, UD_VECTOR);
-
-		if (sev_es_guest(vcpu->kvm)) {
-			/* Perform SEV-ES specific VMCB updates */
-			sev_es_init_vmcb(svm);
-		}
-	}
+	if (sev_guest(vcpu->kvm))
+		sev_init_vmcb(svm);

 	svm_hv_init_vmcb(vmcb);
 	init_vmcb_after_set_cpuid(vcpu);
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 128993feb4c6..444a7a67122a 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -653,10 +653,10 @@ void __init sev_set_cpu_caps(void);
 void __init sev_hardware_setup(void);
 void sev_hardware_unsetup(void);
 int sev_cpu_init(struct svm_cpu_data *sd);
+void sev_init_vmcb(struct vcpu_svm *svm);
 void sev_free_vcpu(struct kvm_vcpu *vcpu);
 int sev_handle_vmgexit(struct kvm_vcpu *vcpu);
 int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in);
-void sev_es_init_vmcb(struct vcpu_svm *svm);
 void sev_es_vcpu_reset(struct vcpu_svm *svm);
 void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
 void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa);

base-commit: 8baacf67c76c560fed954ac972b63e6e59a6fba0
--