qemu: Fix probing of AMD SEV support

So the procedure to detect SEV support works like this:
1) we detect that sev-guest is among the QOM types and set the cap flag
2) we probe the monitor for SEV support
    - this is tricky, because QEMU with compiled SEV support will always
    report -object sev-guest and query-sev-capabilities command, that
    however doesn't mean SEV is supported
3) depending on what the monitor returned, we either keep or clear the
capability flag for SEV

Commit a349c6c21c6 added an explicit check for "GenericError" in the
monitor reply to prevent libvirtd to spam logs about missing
'query-sev-capabilities' command. At the same time though, it returned
success in this case which means that we didn't clear the capability
flag afterwards and happily formatted SEV into qemuCaps. Therefore,
adjust all the relevant callers to handle -1 on errors, 0 on SEV being
unsupported and 1 on SEV being supported.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>

src/qemu/qemu_capabilities.c

@@ -2767,18 +2767,20 @@ virQEMUCapsProbeQMPGICCapabilities(virQEMUCapsPtr qemuCaps,
 }
 
 
+/* Returns -1 on error, 0 if SEV is not supported, 1 if SEV is supported */
 static int
 virQEMUCapsProbeQMPSEVCapabilities(virQEMUCapsPtr qemuCaps,
                                    qemuMonitorPtr mon)
 {
+    int rc = -1;
     virSEVCapability *caps = NULL;
 
-    if (qemuMonitorGetSEVCapabilities(mon, &caps) < 0)
-        return -1;
+    if ((rc = qemuMonitorGetSEVCapabilities(mon, &caps)) <= 0)
+        return rc;
 
     virSEVCapabilitiesFree(qemuCaps->sevCapabilities);
     qemuCaps->sevCapabilities = caps;
-    return 0;
+    return rc;
 }
 
 
@@ -4188,7 +4190,12 @@ virQEMUCapsInitQMPMonitor(virQEMUCapsPtr qemuCaps,
 
     /* Probe for SEV capabilities */
     if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_SEV_GUEST)) {
-        if (virQEMUCapsProbeQMPSEVCapabilities(qemuCaps, mon) < 0)
+        int rc = virQEMUCapsProbeQMPSEVCapabilities(qemuCaps, mon);
+
+        if (rc < 0)
+            goto cleanup;
+
+        if (rc == 0)
             virQEMUCapsClear(qemuCaps, QEMU_CAPS_SEV_GUEST);
     }
 

src/qemu/qemu_monitor_json.c

@@ -6435,6 +6435,20 @@ qemuMonitorJSONGetGICCapabilities(qemuMonitorPtr mon,
 }
 
 
+/**
+ * qemuMonitorJSONGetSEVCapabilities:
+ * @mon: qemu monitor object
+ * @capabilities: pointer to pointer to a SEV capability structure to be filled
+ *
+ * This function queries and fills in AMD's SEV platform-specific data.
+ * Note that from QEMU's POV both -object sev-guest and query-sev-capabilities
+ * can be present even if SEV is not available, which basically leaves us with
+ * checking for JSON "GenericError" in order to differentiate between
+ * compiled-in support and actual SEV support on the platform.
+ *
+ * Returns -1 on error, 0 if SEV is not supported, and 1 if SEV is supported on
+ * the platform.
+ */
 int
 qemuMonitorJSONGetSEVCapabilities(qemuMonitorPtr mon,
                                   virSEVCapability **capabilities)
@@ -6458,8 +6472,7 @@ qemuMonitorJSONGetSEVCapabilities(qemuMonitorPtr mon,
     if (qemuMonitorJSONCommand(mon, cmd, &reply) < 0)
         goto cleanup;
 
-    /* Both -object sev-guest and query-sev-capabilities can be present
-     * even if SEV is not available */
+    /* QEMU has only compiled-in support of SEV */
     if (qemuMonitorJSONHasError(reply, "GenericError")) {
         ret = 0;
         goto cleanup;
@@ -6511,8 +6524,7 @@ qemuMonitorJSONGetSEVCapabilities(qemuMonitorPtr mon,
     capability->cbitpos = cbitpos;
     capability->reduced_phys_bits = reduced_phys_bits;
     VIR_STEAL_PTR(*capabilities, capability);
-    ret = 0;
-
+    ret = 1;
  cleanup:
     virJSONValueFree(cmd);
     virJSONValueFree(reply);

tests/qemucapabilitiesdata/caps_3.0.0.x86_64.xml

@@ -211,7 +211,6 @@
   <flag name='tpm-emulator'/>
   <flag name='mch'/>
   <flag name='mch.extended-tseg-mbytes'/>
-  <flag name='sev-guest'/>
   <flag name='usb-storage.werror'/>
   <flag name='egl-headless'/>
   <flag name='vfio-pci.display'/>