diff options
| -rw-r--r-- | hw/arm/virt.c | 37 |
1 files changed, 33 insertions, 4 deletions
diff --git a/hw/arm/virt.c b/hw/arm/virt.c index 235344034d..34d9379032 100644 --- a/hw/arm/virt.c +++ b/hw/arm/virt.c @@ -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; |