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@xxxxxxx>
---
qapi/qom.json | 11 ++++++++++-
target/i386/sev.c | 18 +++++++++++++++++-
2 files changed, 27 insertions(+), 2 deletions(-)
diff --git a/qapi/qom.json b/qapi/qom.json
index baae3a183f..8f2f75bde6 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 9dab4060b8..f4ee317cb0 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
--
2.25.1
