i386/sev: Fallback to the default SEV device if none provided in sev_get_capabilities()

When management tools (e.g. libvirt) query QEMU capabilities,
they start QEMU with a minimalistic configuration and issue
various commands on monitor. One of the command issued is/might
be "query-sev-capabilities" to learn values like cbitpos or
reduced-phys-bits. But as of v9.0.0-1145-g16dcf200dc the monitor
command returns an error instead.

This creates a chicken-egg problem because in order to query
those aforementioned values QEMU needs to be started with a
'sev-guest' object. But to start QEMU with the values must be
known.

I think it's safe to assume that the default path ("/dev/sev")
provides the same data as user provided one. So fall back to it.

Fixes: 16dcf200dc951c1cde3e5b442457db5f690b8cf0
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Link: https://lore.kernel.org/r/157f93712c23818be193ce785f648f0060b33dee.1719218926.git.mprivozn@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/target/i386/sev.c b/target/i386/sev.c
index 054366878aa2bb3acf6aeaf61debb1d6b2febc08..2f3dbe289f47379f3ed1db424c4401edbd781751 100644 (file)
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -587,13 +587,13 @@ static SevCapability *sev_get_capabilities(Error **errp)
     }
 
     sev_common = SEV_COMMON(MACHINE(qdev_get_machine())->cgs);
-    if (!sev_common) {
-        error_setg(errp, "SEV is not configured");
-        return NULL;
+    if (sev_common) {
+        sev_device = object_property_get_str(OBJECT(sev_common), "sev-device",
+                                             &error_abort);
+    } else {
+        sev_device = g_strdup(DEFAULT_SEV_DEVICE);
     }
 
-    sev_device = object_property_get_str(OBJECT(sev_common), "sev-device",
-                                         &error_abort);
     fd = open(sev_device, O_RDWR);
     if (fd < 0) {
         error_setg_errno(errp, errno, "SEV: Failed to open %s",