[PATCH v8 07/11] libxl: introduce libxl__alloc_vdev

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

Introduce libxl__alloc_vdev: find a spare virtual block device in the
domain passed as argument.


Changes in v7:
- remove the upper bound and document why.

Changes in v6:
- more comments in libxl__devid_to_localdev;
- inline GCSPRINTF.

Changes in v5:
- remove domid paramter to libxl__alloc_vdev (assume
  LIBXL_TOOLSTACK_DOMID);
- remove scaling limit from libxl__alloc_vdev.

Changes in v4:
- rename libxl__devid_to_vdev to libxl__devid_to_localdev;
- introduce upper bound for encode_disk_name;
- better error handling;

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
 tools/libxl/libxl.c          |   32 +++++++++++++++++++++++++
 tools/libxl/libxl_internal.h |    4 +++
 tools/libxl/libxl_linux.c    |   52 ++++++++++++++++++++++++++++++++++++++++++
 tools/libxl/libxl_netbsd.c   |    6 +++++
 4 files changed, 94 insertions(+), 0 deletions(-)

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 1a01b24..3cf0d53 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -1743,6 +1743,38 @@ out:
     return ret;
 }
 
+/* libxl__alloc_vdev only works on the local domain, that is the domain
+ * where the toolstack is running */
+static char * libxl__alloc_vdev(libxl__gc *gc, const char *blkdev_start,
+        xs_transaction_t t)
+{
+    int devid = 0, disk = 0, part = 0;
+    char *dompath = libxl__xs_get_dompath(gc, LIBXL_TOOLSTACK_DOMID);
+
+    libxl__device_disk_dev_number(blkdev_start, &disk, &part);
+    if (part != 0) {
+        LOG(ERROR, "blkdev_start is invalid");
+        return NULL;
+    }
+
+    do {
+        devid = libxl__device_disk_dev_number(GCSPRINTF("d%dp0", disk),
+                NULL, NULL);
+        if (devid < 0)
+            return NULL;
+        if (libxl__xs_read(gc, t,
+                    libxl__sprintf(gc, "%s/device/vbd/%d/backend",
+                        dompath, devid)) == NULL) {
+            if (errno == ENOENT)
+                return libxl__devid_to_localdev(gc, devid);
+            else
+                return NULL;
+        }
+        disk++;
+    } while (1);
+    return NULL;
+}
+
 char * libxl__device_disk_local_attach(libxl__gc *gc,
         const libxl_device_disk *in_disk,
         libxl_device_disk *disk,
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 4be99ca..337ce28 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -78,6 +78,8 @@
 #define LIBXL_PV_EXTRA_MEMORY 1024
 #define LIBXL_HVM_EXTRA_MEMORY 2048
 #define LIBXL_MIN_DOM0_MEM (128*1024)
+/* use 0 as the domid of the toolstack domain for now */
+#define LIBXL_TOOLSTACK_DOMID 0
 #define QEMU_SIGNATURE "DeviceModelRecord0002"
 #define STUBDOM_CONSOLE_LOGGING 0
 #define STUBDOM_CONSOLE_SAVE 1
@@ -915,6 +917,8 @@ static inline void libxl__domaindeathcheck_stop(libxl__gc *gc,
 _hidden int libxl__try_phy_backend(mode_t st_mode);
 
 
+_hidden char *libxl__devid_to_localdev(libxl__gc *gc, int devid);
+
 /* from libxl_pci */
 
 _hidden int libxl__device_pci_add(libxl__gc *gc, uint32_t domid, libxl_device_pci *pcidev, int starting);
diff --git a/tools/libxl/libxl_linux.c b/tools/libxl/libxl_linux.c
index 925248b..0169b2f 100644
--- a/tools/libxl/libxl_linux.c
+++ b/tools/libxl/libxl_linux.c
@@ -25,3 +25,55 @@ int libxl__try_phy_backend(mode_t st_mode)
 
     return 1;
 }
+
+#define EXT_SHIFT 28
+#define EXTENDED (1<<EXT_SHIFT)
+#define VDEV_IS_EXTENDED(dev) ((dev)&(EXTENDED))
+#define BLKIF_MINOR_EXT(dev) ((dev)&(~EXTENDED))
+/* the size of the buffer to store the device name is 32 bytes to match the
+ * equivalent buffer in the Linux kernel code */
+#define BUFFER_SIZE 32
+
+/* Same as in Linux.
+ * encode_disk_name might end up using up to 29 bytes (BUFFER_SIZE - 3)
+ * including the trailing \0.
+ *
+ * The code is safe because 26 raised to the power of 28 (that is the
+ * maximum offset that can be stored in the allocated buffer as a
+ * string) is far greater than UINT_MAX on 64 bits so offset cannot be
+ * big enough to exhaust the available bytes in ret. */
+static char *encode_disk_name(char *ptr, unsigned int n)
+{
+    if (n >= 26)
+        ptr = encode_disk_name(ptr, n / 26 - 1);
+    *ptr = 'a' + n % 26;
+    return ptr + 1;
+}
+
+char *libxl__devid_to_localdev(libxl__gc *gc, int devid)
+{
+    unsigned int minor;
+    int offset;
+    int nr_parts;
+    char *ptr = NULL;
+    char *ret = libxl__zalloc(gc, BUFFER_SIZE);
+
+    if (!VDEV_IS_EXTENDED(devid)) {
+        minor = devid & 0xff;
+        nr_parts = 16;
+    } else {
+        minor = BLKIF_MINOR_EXT(devid);
+        nr_parts = 256;
+    }
+    offset = minor / nr_parts;
+
+    strcpy(ret, "xvd");
+    ptr = encode_disk_name(ret + 3, offset);
+    if (minor % nr_parts == 0)
+        *ptr = 0;
+    else
+        /* overflow cannot happen, thanks to the upper bound */
+        snprintf(ptr, ret + 32 - ptr,
+                "%d", minor & (nr_parts - 1));
+    return ret;
+}
diff --git a/tools/libxl/libxl_netbsd.c b/tools/libxl/libxl_netbsd.c
index 9e0ed6d..dbf5f71 100644
--- a/tools/libxl/libxl_netbsd.c
+++ b/tools/libxl/libxl_netbsd.c
@@ -24,3 +24,9 @@ int libxl__try_phy_backend(mode_t st_mode)
 
     return 0;
 }
+
+char *libxl__devid_to_localdev(libxl__gc *gc, int devid)
+{
+    /* TODO */
+    return NULL;
+}
-- 
1.7.2.5