[Xen-devel] [PATCH v3 7/9] libxl: Allow local access for block devices with hotplug scripts

[George Dunlap <george.dunlap@xxxxxxxxxx>]

From: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

pygrub and qemuu need to be able to access a VM's disks locally in
order to be able to pull out the kernel and provide emulated disk
access, respectively.  This can be done either by accessing the local
disk directly, or by plugging the target disk into dom0 to allow
access.

Unfortunately, while the plugging machinery works for pygrub, it does
not yet work for qemuu; this means that at the moment, disks with
hotplug scripts or disks with non-dom0 backends cannot be provided as
emulated devices to HVM domains.

Fortunately, disks using hotplug scripts created in dom0 do create a
block device as part of set-up, which can be accessed locally; and if
they use block-common.sh:write_dev, this path will be written to
physical-device-path.

Modify libxl__device_disk_setdefault() to be able to fish this path
out of xenstore and pass it to the caller.

Unfortunately, at the time pygrub runs, the devices have not yet been
set up.  Rather than try to stash the domid somewhere to pass, we just
pass INVALID_DOMID.

This allows qemuu to emulate block devices created with custom hotplug
scripts.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
---
Changes since v1:
- Ported on top of previous changed patches
- Removed vestigal paragraph from changelog
- Change domid argument to guest_domid
- Added a missing space after an if ()
- Trimmed some long lines

CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monne <roger.pau@xxxxxxxxxx>
---
 tools/libxl/libxl.c          | 43 ++++++++++++++++++++++++++++++++++++++-----
 tools/libxl/libxl_dm.c       |  4 ++--
 tools/libxl/libxl_internal.h |  3 ++-
 3 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 602bec2..944f4d1 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -2295,7 +2295,7 @@ int libxl__device_disk_setdefault(libxl__gc *gc, 
libxl_device_disk *disk)
 }
 
 int libxl__device_from_disk(libxl__gc *gc, uint32_t domid,
-                                   libxl_device_disk *disk,
+                                   const libxl_device_disk *disk,
                                    libxl__device *device)
 {
     int devid;
@@ -3001,8 +3001,9 @@ static char * libxl__alloc_vdev(libxl__gc *gc, void 
*get_vdev_user,
 /* Callbacks */
 
 char *libxl__device_disk_find_local_path(libxl__gc *gc, 
-                                         const libxl_device_disk *disk,
-                                         bool qdisk_direct)
+                                          libxl_domid guest_domid,
+                                          const libxl_device_disk *disk,
+                                          bool qdisk_direct)
 {
     char *path = NULL;
 
@@ -3031,6 +3032,38 @@ char *libxl__device_disk_find_local_path(libxl__gc *gc,
         path = libxl__strdup(gc, disk->pdev_path);
         LOG(DEBUG, "Directly accessing local QDISK target %s", path);
         goto out;
+    } 
+
+    /* 
+     * If the format isn't raw and / or we're using a script, then see
+     * if the script has written a path to the "cooked" node
+     */
+    if (disk->script && guest_domid != INVALID_DOMID) {
+        libxl__device device;
+        char *be_path, *pdpath;
+        int rc;
+
+        LOG(DEBUG,
+            "Run from a script; checking for physical-device-path (vdev %s)",
+            disk->vdev);
+
+        rc = libxl__device_from_disk(gc, guest_domid, disk, &device);
+        if (rc < 0)
+            goto out;
+
+        be_path = libxl__device_backend_path(gc, &device);
+
+        pdpath = libxl__sprintf(gc, "%s/physical-device-path", be_path);
+
+        LOG(DEBUG, "Attempting to read node %s", pdpath);
+        path = libxl__xs_read(gc, XBT_NULL, pdpath);
+
+        if (path)
+            LOG(DEBUG, "Accessing cooked block device %s", path);
+        else
+            LOG(DEBUG, "No physical-device-path, can't access locally.");
+
+        goto out;
     }
 
  out:
@@ -3057,7 +3090,8 @@ void libxl__device_disk_local_initiate_attach(libxl__egc 
*egc,
 
     LOG(DEBUG, "Trying to find local path");
 
-    dls->diskpath = libxl__device_disk_find_local_path(gc, in_disk, false);
+    dls->diskpath = libxl__device_disk_find_local_path(gc, INVALID_DOMID,
+                                                       in_disk, false);
     if (dls->diskpath) {
         LOG(DEBUG, "Local path found, executing callback.");
         dls->callback(egc, dls, 0);
@@ -3073,7 +3107,6 @@ void libxl__device_disk_local_initiate_attach(libxl__egc 
*egc,
         rc = libxl__device_disk_setdefault(gc, disk);
         if (rc) goto out;
 
-        /* If we can't find a local path, attach it */
         libxl__prepare_ao_device(ao, &dls->aodev);
         dls->aodev.callback = local_device_attach_cb;
         device_disk_add(egc, LIBXL_TOOLSTACK_DOMID, disk, &dls->aodev,
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index 2eef53a..07559b6 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -1214,8 +1214,8 @@ static int libxl__build_device_model_args_new(libxl__gc 
*gc,
                     target_path = libxl__blktap_devpath(gc, disks[i].pdev_path,
                                                         disks[i].format);
                 else
-                    target_path = libxl__device_disk_find_local_path(gc, 
-                                                            &disks[i], true);
+                    target_path = libxl__device_disk_find_local_path(gc,
+                                                 guest_domid, &disks[i], true);
 
                 if (!target_path) {
                     LOG(WARN, "No way to get local access disk to image: %s\n"
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 268a2f4..9fde212 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -1712,7 +1712,7 @@ _hidden char *libxl__blktap_devpath(libxl__gc *gc,
 _hidden int libxl__device_destroy_tapdisk(libxl__gc *gc, const char *params);
 
 _hidden int libxl__device_from_disk(libxl__gc *gc, uint32_t domid,
-                                   libxl_device_disk *disk,
+                                   const libxl_device_disk *disk,
                                    libxl__device *device);
 
 /* Calls poll() again - useful to check whether a signaled condition
@@ -2692,6 +2692,7 @@ static inline void 
libxl__device_disk_local_init(libxl__disk_local_state *dls)
  * See if we can find a way to access a disk locally
  */
 _hidden char * libxl__device_disk_find_local_path(libxl__gc *gc, 
+                                                  libxl_domid guest_domid,
                                                   const libxl_device_disk 
*disk,
                                                   bool qdisk_direct);
 
-- 
2.1.4


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