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[Qemu-devel] [PATCH] hw/arm/virt: explain device-to-transport mapping in
From: |
Laszlo Ersek |
Subject: |
[Qemu-devel] [PATCH] hw/arm/virt: explain device-to-transport mapping in create_virtio_devices() |
Date: |
Fri, 30 Jan 2015 05:31:13 +0100 |
Signed-off-by: Laszlo Ersek <address@hidden>
---
hw/arm/virt.c | 32 ++++++++++++++++++++++++++++----
1 file changed, 28 insertions(+), 4 deletions(-)
diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 2353440..091e5ee 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -441,10 +441,27 @@ 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 reserved; 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.
*/
for (i = 0; i < NUM_VIRTIO_TRANSPORTS; i++) {
int irq = vbi->irqmap[VIRT_MMIO] + i;
@@ -453,6 +470,13 @@ 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;
--
1.8.3.1
- [Qemu-devel] [PATCH] hw/arm/virt: explain device-to-transport mapping in create_virtio_devices(),
Laszlo Ersek <=