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hw/arm/virt: explain device-to-transport mapping in create_virtio_devices()

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Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1422592273-4432-1-git-send-email-lersek@redhat.com
[PMM: added note recommending UUIDs]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
This commit is contained in:
Laszlo Ersek 2015-02-05 13:37:23 +00:00 committed by Peter Maydell
parent 45140a5767
commit 587078f0ed
1 changed files with 33 additions and 4 deletions
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37
hw/arm/virt.c
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@ -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;