On Sat, 2025-03-22 at 01:29 +0100, Halil Pasic wrote:

As per virtio spec the fields cols and rows are specified as little endian. Although there is no legacy interface requirement that would state that cols and rows need to be handled as native endian when legacy interface is used, unlike for the fields of the adjacent struct virtio_console_control, I decided to err on the side of caution based on some non-conclusive virtio spec repo archaeology and opt for using virtio16_to_cpu() much like for virtio_console_control.event. Strictly by the letter of the spec virtio_le_to_cpu() would have been sufficient. But when the legacy interface is not used, it boils down to the same.

And when using the legacy interface, the device formatting these as little endian when the guest is big endian would surprise me more than it using guest native byte order (which would make it compatible with the current implementation). Nevertheless somebody trying to implement the spec following it to the letter could end up forcing little endian byte order when the legacy interface is in use. So IMHO this ultimately needs a judgement call by the maintainers.

Fixes: 8345adbf96fc1 ("virtio: console: Accept console size along with resize control message") Signed-off-by: Halil Pasic pasic@linux.ibm.com Cc: stable@vger.kernel.org # v2.6.35+

---

@Michael: I think it would be nice to add a clarification on the byte order to be used for cols and rows when the legacy interface is used to the spec, regardless of what we decide the right byte order is. If it is native endian that shall be stated much like it is stated for virtio_console_control. If it is little endian, I would like to add a sentence that states that unlike for the fields of virtio_console_control the byte order of the fields of struct virtio_console_resize is little endian also when the legacy interface is used.

@Maximilian: Would you mind giving this a spin with your implementation on the device side of things in QEMU?

---

drivers/char/virtio_console.c | 7 ++++---  1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c index 18f92dd44d45..fc698e2b1da1 100644 --- a/drivers/char/virtio_console.c +++ b/drivers/char/virtio_console.c @@ -1579,8 +1579,8 @@ static void handle_control_message(struct virtio_device *vdev,   break;   case VIRTIO_CONSOLE_RESIZE: {   struct {

• 	__u16 rows;
  

• 	__u16 cols;
  

• 	__virtio16 rows;
  

• 	__virtio16 cols;
  

} size;     if (!is_console_port(port)) @@ -1588,7 +1588,8 @@ static void handle_control_message(struct virtio_device *vdev,     memcpy(&size, buf->buf + buf->offset + sizeof(*cpkt),          sizeof(size));

• set_console_size(port, size.rows, size.cols);
  

• set_console_size(port, virtio16_to_cpu(vdev,
  

size.rows),

• 		 virtio16_to_cpu(vdev, size.cols));
  

port->cons.hvc->irq_requested = 1;   resize_console(port);

base-commit: b3ee1e4609512dfff642a96b34d7e5dfcdc92d05

It took me a while to recompile the kernel, but now that it has compiled it works! Unfortunately, images don't lend themselves well to mailing lists, but here is tmux running at 18x55(you'll just have to trust me that it's over a virtio serial console)

│top - 12:54:04 up 4 min, 1 ~ │Tasks: 222 total, 1 runni ~ │%Cpu(s): 0.0 us, 0.0 sy, ~ │MiB Mem : 15987.2 total, ~ │MiB Swap: 8192.0 total, ~ │ ~ │ PID USER PR NI ~ │ 1 root 20 0 ~ │ 2 root 20 0 ~ │ 3 root 20 0 ~ │ 4 root 0 -20 ~ │ 5 root 0 -20 ~ │ 6 root 0 -20 ~ │ 7 root 0 -20 ~ │ 8 root 0 -20 [No Name] 0,0-1 All│ 9 root 20 0 │ 10 root 0 -20 [0] 0:zsh* "fedora" 12:53 24-Mar-25

Cheers, Max