[xen staging] libxl: Support blktap with HVM device model

[patchbot@xxxxxxx]

commit d7c4d59b51929451a047e0fb62b229c67d6dd7be
Author:     Jason Andryuk <jandryuk@xxxxxxxxx>
AuthorDate: Thu Apr 25 09:47:52 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Apr 25 09:47:52 2024 +0200

    libxl: Support blktap with HVM device model
    
    blktap exposes disks over UNIX socket Network Block Device (NBD).
    Modify libxl__device_disk_find_local_path() to provide back the
    QEMU-formatted NBD path.  This allows tapdisk to be used for booting an
    HVM.
    
    Use the nbd+unix:/// format specified by the protocol at
    https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.md
    
    Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>
    Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
    Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
---
 tools/libs/light/libxl_disk.c | 18 ++++++++++++++----
 tools/libs/light/libxl_dm.c   |  1 -
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/tools/libs/light/libxl_disk.c b/tools/libs/light/libxl_disk.c
index fa7856f28c..1f1e5c6567 100644
--- a/tools/libs/light/libxl_disk.c
+++ b/tools/libs/light/libxl_disk.c
@@ -1361,7 +1361,8 @@ char *libxl__device_disk_find_local_path(libxl__gc *gc,
      * 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) {
+    if ((disk->script && guest_domid != INVALID_DOMID) ||
+        disk->backend == LIBXL_DISK_BACKEND_TAP) {
         libxl__device device;
         char *be_path, *pdpath;
         int rc;
@@ -1381,10 +1382,19 @@ char *libxl__device_disk_find_local_path(libxl__gc *gc,
         LOGD(DEBUG, guest_domid, "Attempting to read node %s", pdpath);
         path = libxl__xs_read(gc, XBT_NULL, pdpath);
 
-        if (path)
+        if (path) {
             LOGD(DEBUG, guest_domid, "Accessing cooked block device %s", path);
-        else
-            LOGD(DEBUG, guest_domid, "No physical-device-path, can't access 
locally.");
+
+            /* tapdisk exposes disks locally over UNIX socket NBD. */
+            if (disk->backend == LIBXL_DISK_BACKEND_TAP) {
+                path = libxl__sprintf(gc, "nbd+unix:///?socket=%s", path);
+                LOGD(DEBUG, guest_domid,
+                     "Directly accessing local TAP target %s", path);
+            }
+        } else {
+            LOGD(DEBUG, guest_domid,
+                "No physical-device-path, can't access locally.");
+        }
 
         goto out;
     }
diff --git a/tools/libs/light/libxl_dm.c b/tools/libs/light/libxl_dm.c
index 567ae1ec7e..ff8ddeec9a 100644
--- a/tools/libs/light/libxl_dm.c
+++ b/tools/libs/light/libxl_dm.c
@@ -1915,7 +1915,6 @@ static int libxl__build_device_model_args_new(libxl__gc 
*gc,
                     continue;
                 }
 
-                assert(disks[i].backend != LIBXL_DISK_BACKEND_TAP);
                 target_path = libxl__device_disk_find_local_path(gc,
                                     guest_domid, &disks[i], true);
 
--
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