[Qemu-devel] ACPI PCI hotplug table updates

[open sorcerer <0p3n.s0rc3r3r@gmail.com>]

Hi,

I am digging into an issue where qmp_device_del does not actually delete
devices when a guest OS is in prelaunch. This seems to be due to the guest
OS not handling ACPI events because it is not currently running. If I
assume correctly, qmp should allow you to add/remove devices while the host
is down, or if not possible, publish an error message.

I think fixing this issue is as simple as making sure that the VM is in a
safe state to ignore the hotplug ACPI dance but eject the disk, something
like:

prelaunch, preconfig, shutdown: ignore acpi and deal with cleaning devices
other non-running: bubble up error
running: default behavior

I was trying to validate that this change would be safe (keep in mind I am
learning ACPI in little pieces while digging) using GDB, and code
inspection. While stepping through with GDB i noticed that the PCI slots
are controlled by memory region and the opaque acpi pci hp state object. I
was unable this far to find any code executed that modifies the ACPI tables
beyond just the pci hotplug state.

I also tried to test using "while true; do acpidump | md5; sleep 1; done"
in the guest OS and then add/remove a virtio-blk-pci device (which
exercised the ACPI callbacks via piix4 callbacks). The output of the
acpidump -> md5 was consistent during each phase of the data collection
which I believe implied that the acpi tables were not modified by the PCI
hotplug.

Can someone help me understand:

1. Are the ACPI tables not modified when doing PCI hotplug?
2. Do the general changes proposed seem safe?
3. Are there resources or documentation I can read to help me understand
this problem further? I have skimmed through alot of different documents
and watched some youtube videos, but the ACPI documentation is hard to read
and sift through and the youtube videos are generally too high level.

Thanks.