ACPI in qemu


I’m fiddling around with QEMU and trying to get graceful shutdown working, triggered from outside the VM. Here’s what I figured out:

Part 1: QMP

QEMU has a thing called “QMP”, which I assume stands for “QEMU Management Protocol”. It will listen on a socket and you can chat with a running VM to control it a bit.

Run the VM with -qmp unix:/tmp/qmp.sock,server,nowait, then you can talk to that socket. I’m sure there are libraries for this, but it looks like if you send the messages {"execute": "qmp_capabilities"} and then {"execute": "system_powerdown"}, it does what I want.

c ='/tmp/qmp.sock')
c.puts('{"execute": "qmp_capabilities"}')
c.puts('{"execute": "system_powerdown"}')

This sends a message via ACPI to acpid running in my initramfs. In order for that to work, I have to set up /dev/input/event0:

mknod /dev/input/event0 c 13 64

Then, when I run that QMP script above, acpid executes the file at /etc/acpi/PWRF/00000080.

Part 2: Port I/O

So what do we do in that ACPI event script? Well, first, we’ll want to fsync, shut down processes, whatever, but at the end of the day I want the QEMU process to terminate. Here’s how we do it:

#include <sys/io.h>
#include <stdio.h>

int main() {
  printf ("Powering off...\n");

  /* In modern versions of qemu, in order to terminate the VM from inside,
   * we have to run qemu with `-device isa-debug-exit` and write a message
   * (0x31) to a port (0x501). Port I/O is a weird kernel-y thing, but just realize that:
   *   - ioperm gives us permission to write to port 0x501
   *   - outb writes a byte (0x31) to that port.
   * The byte we pass is doubled and incremented to build an exit code. So, we
   * can't end up with a clean (0) exit, or an even number. We pass 0x31=49 to end
   * up with an exit code of 49*2+1 = 99.
  ioperm(0x501, 1, 1);
  outb(0x31, 0x501);

  printf ("failed to shut down...\n");
  for (;;) { }

So we compile that (I’m using an alpine docker image and compiling with -O -static), put it in the initramfs at /sbin/acpi-shutdown, and write a script at /etc/acpi/PWRF/00000080 that finishes by calling it, and invoke qemu with -device isa-debug-exit.

After all that, when we run the script to send the QMP event for machine shutdown, this binary executes and causes QEMU to actually terminate with exit status 99 after doing whatever graceful termination seems useful.