@@ -441,10 +441,32 @@ static void create_virtio_devices(const VirtBoardInfo *vbi, qemu_irq *pic)
int i;
hwaddr size = vbi->memmap[VIRT_MMIO].size;
- /* Note that we have to create the transports in forwards order
- * so that command line devices are inserted lowest address first,
- * and then add dtb nodes in reverse order so that they appear in
- * the finished device tree lowest address first.
+ /* We create the transports in forwards order. Since qbus_realize()
+ * prepends (not appends) new child buses, the incrementing loop below will
+ * create a list of virtio-mmio buses with decreasing base addresses.
+ *
+ * When a -device option is processed from the command line,
+ * qbus_find_recursive() picks the next free virtio-mmio bus in forwards
+ * order. The upshot is that -device options in increasing command line
+ * order are mapped to virtio-mmio buses with decreasing base addresses.
+ *
+ * When this code was originally written, that arrangement ensured that the
+ * guest Linux kernel would give the lowest "name" (/dev/vda, eth0, etc) to
+ * the first -device on the command line. (The end-to-end order is a
+ * function of this loop, qbus_realize(), qbus_find_recursive(), and the
+ * guest kernel's name-to-address assignment strategy.)
+ *
+ * Meanwhile, the kernel's traversal seems to have been reversed; see eg.
+ * the message, if not necessarily the code, of commit 70161ff336.
+ * Therefore the loop now establishes the inverse of the original intent.
+ *
+ * Unfortunately, we can't counteract the kernel change by reversing the
+ * loop; it would break existing command lines.
+ *
+ * In any case, the kernel makes no guarantee about the stability of
+ * enumeration order of virtio devices (as demonstrated by it changing
+ * between kernel versions). For reliable and stable identification
+ * of disks users must use UUIDs or similar mechanisms.
*/
for (i = 0; i < NUM_VIRTIO_TRANSPORTS; i++) {
int irq = vbi->irqmap[VIRT_MMIO] + i;
@@ -453,6 +475,13 @@ static void create_virtio_devices(const VirtBoardInfo *vbi, qemu_irq *pic)
sysbus_create_simple("virtio-mmio", base, pic[irq]);
}
+ /* We add dtb nodes in reverse order so that they appear in the finished
+ * device tree lowest address first.
+ *
+ * Note that this mapping is independent of the loop above. The previous
+ * loop influences virtio device to virtio transport assignment, whereas
+ * this loop controls how virtio transports are laid out in the dtb.
+ */
for (i = NUM_VIRTIO_TRANSPORTS - 1; i >= 0; i--) {
char *nodename;
int irq = vbi->irqmap[VIRT_MMIO] + i;