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@gmail.com>
---
 tools/libs/light/libxl_disk.c | 17 +++++++++++++----
 tools/libs/light/libxl_dm.c   |  1 -
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/tools/libs/light/libxl_disk.c b/tools/libs/light/libxl_disk.c
index 59ff996837..b65cad33cc 100644
--- a/tools/libs/light/libxl_disk.c
+++ b/tools/libs/light/libxl_disk.c
@@ -1317,7 +1317,8 @@ 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;
@@ -1337,10 +1338,18 @@ 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 14b593110f..f7c796011d 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);
 
-- 
2.43.0

On Thu, Feb 01, 2024 at 01:30:24PM -0500, Jason Andryuk wrote:
> diff --git a/tools/libs/light/libxl_disk.c b/tools/libs/light/libxl_disk.c
> @@ -1337,10 +1338,18 @@ 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.");

Here, the coding style call to have both side of the if..else to have { }
or none of them. Could you add {}-block for the else side?


Beside that, patch looks fine to me:
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

Thanks,

-- 
Anthony PERARD