[PATCH 26/28] libxl: spawns two QEMUs for HVM guests

[Ian Jackson <ian.jackson@eu.citrix.com>]

From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

We actually need to spawn a second QEMU if we want to run qemu as
non-root, because the pv backends ought to run as root (or device
hotplug may not work).

So in this case, start a second QEMU to provide PV backends in
userspace to HVM guests.  Use both dcs->dmss.pvqemu and dcs->dmss.dm
to keep track of the starting QEMUs.

Only proceed when both QEMUs have started.

And, we only default to running QEMU as non-root if we are going to be
able to run split qemus.  In particular, it is not safe to run split
qemus if they don't support the emulator_id option, because we need to
split the xenstore paths too.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
v6: Split only if trying to run a qemu as non-root
    Reorganise changes to dm callbacks
    No more dcs in dmss
    Use min() to calculate worst rc
    Explicitly set unused fields of dmss.pvqemu to 0
    Change error handling

v3: use dcs->dmss.pvqemu to spawn the second QEMU
    keep track of the rc of both QEMUs before proceeding
---
 tools/libxl/libxl_create.c |   72 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)

diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index e67e402..59dfcd67 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -431,6 +431,8 @@ static int domcreate_setdefault_dm_user(libxl__gc *gc,
     int rc;
     const char *user;
 
+    const char *dm = libxl__domain_device_model(gc, b_info);
+
     if (b_info->device_model_user)
         /* already set, good-oh */
         return 0;
@@ -440,6 +442,15 @@ static int domcreate_setdefault_dm_user(libxl__gc *gc,
         /* we're not going to run it anyway */
         return 0;
 
+    if (!libxl__dm_supported(gc, dm, libxl__dm_support_check__emulator_id)) {
+        /* we don't want to run the pv backends as non-root because
+         * device hotplug will no longer work. */
+        LOG(WARN,
+ "Device model does not support split PV backends, running it as root");
+        user = "root";
+        goto found;
+    }
+
     user = GCSPRINTF("%s%d", LIBXL_QEMU_USER_BASE, domid);
 
     rc = dm_runas_helper(gc, user);
@@ -802,6 +813,14 @@ static void remus_checkpoint_stream_done(
 /* Event callbacks, in this order: */
 static void domcreate_dm_support_checked(libxl__egc *egc,
     libxl__dm_support_check_state *checking, int rc);
+
+static void domcreate_dm_local_split_pv_cb(libxl__egc *egc,
+                                           libxl__dm_spawn_state *dmss);
+static void domcreate_dm_local_split_dm_cb(libxl__egc *egc,
+                                           libxl__dm_spawn_state *dmss);
+static void domcreate_dm_local_split_cb(libxl__egc *egc,
+                                        libxl__domain_create_state *dcs);
+
 static void domcreate_devmodel_started(libxl__egc *egc,
                                        libxl__dm_spawn_state *dmss);
 static void domcreate_bootloader_console_available(libxl__egc *egc,
@@ -1044,6 +1063,9 @@ static void domcreate_dm_support_checked(libxl__egc *egc,
     
     /* convenience aliases */
     libxl_domain_config *const d_config = dcs->guest_config;
+    const libxl_domain_build_info *b_info = &d_config->b_info;
+    libxl__domain_build_state *const b_state = &dcs->build_state;
+    const uint32_t domid = dcs->guest_domid;
     const int restore_fd = dcs->restore_fd;
 
     if (rc) goto out;
@@ -1053,6 +1075,15 @@ static void domcreate_dm_support_checked(libxl__egc *egc,
     rc = domcreate_setdefault_dm_user(gc, dcs);
     if (rc) goto out;
 
+    /* run two qemus? */
+    if (d_config->c_info.type == LIBXL_DOMAIN_TYPE_HVM &&
+        !libxl_defbool_val(d_config->b_info.device_model_stubdomain) &&
+        b_info->device_model_user &&
+        strcmp(b_info->device_model_user, "root")) {
+        rc = libxl__dm_emuidmap_add(gc, domid, b_state, EMUID_SPLIT);
+        if (rc) goto out;
+    }
+
     dcs->bl.ao = ao;
     libxl_device_disk *bootdisk =
         d_config->num_disks > 0 ? &d_config->disks[0] : NULL;
@@ -1130,6 +1161,11 @@ static void domcreate_bootloader_done(libxl__egc *egc,
     dcs->dmss.dm.callback = domcreate_devmodel_started;
     dcs->dmss.callback = domcreate_devmodel_started;
 
+    dcs->dmss.pvqemu.spawn.ao = ao;
+    dcs->dmss.pvqemu.guest_domid = domid;
+    dcs->dmss.pvqemu.guest_config = 0;
+    dcs->dmss.pvqemu.build_state = 0;
+
     if (restore_fd < 0 && dcs->domid_soft_reset == INVALID_DOMID) {
         rc = libxl__domain_build(gc, d_config, domid, state);
         domcreate_rebuild_done(egc, dcs, rc);
@@ -1398,6 +1434,16 @@ static void domcreate_launch_dm(libxl__egc *egc, libxl__multidev *multidev,
         if (libxl_defbool_val(d_config->b_info.device_model_stubdomain)) {
             libxl__spawn_stub_dm(egc, &dcs->dmss, EMUID_DM);
         } else {
+            if (state->emuidmap & (1u << EMUID_SPLIT)) {
+                dcs->dmss.dm.rc = 1;
+                dcs->dmss.dm.callback = domcreate_dm_local_split_dm_cb;
+
+                dcs->dmss.pvqemu.rc = 1; /* +ve means in progress */
+                dcs->dmss.pvqemu.callback = domcreate_dm_local_split_pv_cb;
+
+                libxl__spawn_qdisk_backend(egc, &dcs->dmss.pvqemu);
+            }
+
             libxl__spawn_local_dm(egc, &dcs->dmss.dm, EMUID_DM);
         }
 
@@ -1455,6 +1501,32 @@ static void domcreate_launch_dm(libxl__egc *egc, libxl__multidev *multidev,
     domcreate_complete(egc, dcs, ret);
 }
 
+static void domcreate_dm_local_split_pv_cb(libxl__egc *egc,
+                                           libxl__dm_spawn_state *dmss)
+{
+    libxl__domain_create_state *dcs = CONTAINER_OF(dmss, *dcs, dmss.pvqemu);
+    domcreate_dm_local_split_cb(egc, dcs);
+}
+
+static void domcreate_dm_local_split_dm_cb(libxl__egc *egc,
+                                           libxl__dm_spawn_state *dmss)
+{
+    libxl__domain_create_state *dcs = CONTAINER_OF(dmss, *dcs, dmss.dm);
+    domcreate_dm_local_split_cb(egc, dcs);
+}
+
+static void domcreate_dm_local_split_cb(libxl__egc *egc,
+                                        libxl__domain_create_state *dcs)
+{
+    if (dcs->dmss.dm.rc > 0 ||
+        dcs->dmss.pvqemu.rc > 0)
+        /* something is still in progress */
+        return;
+
+    dcs->dmss.dm.rc = min(dcs->dmss.dm.rc, dcs->dmss.pvqemu.rc);
+    domcreate_devmodel_started(egc, &dcs->dmss.dm);
+}
+
 static void domcreate_devmodel_started(libxl__egc *egc,
                                        libxl__dm_spawn_state *dmss)
 {
-- 
1.7.10.4