[libvirt] [PATCH] qemu: Warn when using vhost-user without shared memory

[Martin Kletzander]

When user configures vhost-user interface and forgets to also configure
any shared memory, the search for the root cause of non-operational
interface might take unpleasantly long time.  Let's enhance user
experience by emitting a warning in the logs.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1266982

Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
---
 src/qemu/qemu_process.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index 420196264685..fb471342e790 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -4542,6 +4542,7 @@ qemuProcessLaunch(virConnectPtr conn,
     unsigned int hostdev_flags = 0;
     size_t nnicindexes = 0;
     int *nicindexes = NULL;
+    bool check_shmem = false;

     VIR_DEBUG("vm=%p name=%s id=%d asyncJob=%d "
               "incoming.launchURI=%s incoming.deferredURI=%s "
@@ -4749,6 +4750,49 @@ qemuProcessLaunch(virConnectPtr conn,
             goto cleanup;
     }

+    VIR_DEBUG("Checking for any possible (non-fatal) issues");
+
+    /*
+     * For vhost-user to work, the domain has to have some type of
+     * shared memory configured.  We're not the proper once to judge
+     * whether shared hugepages or shm are enough and will be in the
+     * future, so we'll just warn in case there is none of that
+     * configured.  Moreover failing would give the false illusion
+     * that libvirt is really checking that everything works before
+     * running the domain and not only we are unable to do that, but
+     * it's also not our aim to do so.
+     */
+    for (i = 0; i < vm->def->nnets; i++) {
+        if (virDomainNetGetActualType(vm->def->nets[i]) ==
+                                      VIR_DOMAIN_NET_TYPE_VHOSTUSER) {
+            check_shmem = true;
+            break;
+        }
+    }
+
+    if (check_shmem) {
+        bool shmem = vm->def->nshmems;
+
+        /*
+         * This check is by no means complete.  We merely check
+         * whetere there are *some* hugepages enabled and *some* NUMA
+         * nodes with shared memory access.
+         */
+        if (!shmem && vm->def->mem.nhugepages) {
+            for (i = 0; i < virDomainNumaGetNodeCount(vm->def->numa); i++) {
+                if (virDomainNumaGetNodeMemoryAccessMode(vm->def->numa, i) ==
+                    VIR_NUMA_MEM_ACCESS_SHARED)
+                    shmem = true;
+                break;
+            }
+        }
+
+        if (!shmem) {
+            VIR_WARN("Detected vhost-user interface without any shared memory. "
+                     "The interface might not be operational");
+        }
+    }
+
     VIR_DEBUG("Building emulator command line");
     if (!(cmd = qemuBuildCommandLine(conn, driver, vm->def, priv->monConfig,
                                      priv->monJSON, priv->qemuCaps,
-- 
2.6.3