i386/sev: Add 'legacy-vm-type' parameter for SEV guest objects

QEMU will currently automatically make use of the KVM_SEV_INIT2 API for
initializing SEV and SEV-ES guests verses the older
KVM_SEV_INIT/KVM_SEV_ES_INIT interfaces.

However, the older interfaces will silently avoid sync'ing FPU/XSAVE
state to the VMSA prior to encryption, thus relying on behavior and
measurements that assume the related fields to be allow zero.

With KVM_SEV_INIT2, this state is now synced into the VMSA, resulting in
measurements changes and, theoretically, behaviorial changes, though the
latter are unlikely to be seen in practice.

To allow a smooth transition to the newer interface, while still
providing a mechanism to maintain backward compatibility with VMs
created using the older interfaces, provide a new command-line
parameter:

  -object sev-guest,legacy-vm-type=true,...

and have it default to false.

Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-ID: <20240409230743.962513-2-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 27 insertions, 2 deletions

diff --git a/qapi/qom.json b/qapi/qom.json
index 85e6b4f..38dde6d 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -898,6 +898,14 @@
 #     designated guest firmware page for measured boot with -kernel
 #     (default: false) (since 6.2)
 #
+# @legacy-vm-type: Use legacy KVM_SEV_INIT KVM interface for creating the VM.
+#                  The newer KVM_SEV_INIT2 interface syncs additional vCPU
+#                  state when initializing the VMSA structures, which will
+#                  result in a different guest measurement. Set this to
+#                  maintain compatibility with older QEMU or kernel versions
+#                  that rely on legacy KVM_SEV_INIT behavior.
+#                  (default: false) (since 9.1)
+#
 # Since: 2.12
 ##
 { 'struct': 'SevGuestProperties',
@@ -908,7 +916,8 @@
             '*handle': 'uint32',
             '*cbitpos': 'uint32',
             'reduced-phys-bits': 'uint32',
-            '*kernel-hashes': 'bool' } }
+            '*kernel-hashes': 'bool',
+            '*legacy-vm-type': 'bool' } }
 
 ##
 # @ThreadContextProperties:
diff --git a/target/i386/sev.c b/target/i386/sev.c
index 9dab406..f4ee317 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -67,6 +67,7 @@ struct SevGuestState {
     uint32_t cbitpos;
     uint32_t reduced_phys_bits;
     bool kernel_hashes;
+    bool legacy_vm_type;
 
     /* runtime state */
     uint32_t handle;
@@ -356,6 +357,16 @@ static void sev_guest_set_kernel_hashes(Object *obj, bool value, Error **errp)
     sev->kernel_hashes = value;
 }
 
+static bool sev_guest_get_legacy_vm_type(Object *obj, Error **errp)
+{
+    return SEV_GUEST(obj)->legacy_vm_type;
+}
+
+static void sev_guest_set_legacy_vm_type(Object *obj, bool value, Error **errp)
+{
+    SEV_GUEST(obj)->legacy_vm_type = value;
+}
+
 bool
 sev_enabled(void)
 {
@@ -863,7 +874,7 @@ static int sev_kvm_type(X86ConfidentialGuest *cg)
     }
 
     kvm_type = (sev->policy & SEV_POLICY_ES) ? KVM_X86_SEV_ES_VM : KVM_X86_SEV_VM;
-    if (kvm_is_vm_type_supported(kvm_type)) {
+    if (kvm_is_vm_type_supported(kvm_type) && !sev->legacy_vm_type) {
         sev->kvm_type = kvm_type;
     } else {
         sev->kvm_type = KVM_X86_DEFAULT_VM;
@@ -1381,6 +1392,11 @@ sev_guest_class_init(ObjectClass *oc, void *data)
                                    sev_guest_set_kernel_hashes);
     object_class_property_set_description(oc, "kernel-hashes",
             "add kernel hashes to guest firmware for measured Linux boot");
+    object_class_property_add_bool(oc, "legacy-vm-type",
+                                   sev_guest_get_legacy_vm_type,
+                                   sev_guest_set_legacy_vm_type);
+    object_class_property_set_description(oc, "legacy-vm-type",
+            "use legacy VM type to maintain measurement compatibility with older QEMU or kernel versions.");
 }
 
 static void