Re: [Qemu-devel] ACPI PCI hotplug table updates

[Igor Mammedov]

On Wed, 3 Oct 2018 10:44:20 -0700
open sorcerer <address@hidden> wrote:

> 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.
may I ask why one would delete a device at -S pause point, isn't it easier
to start QEMU without it, to begin with?

> 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:
in prelaunch runstate where '-S' option pauses VM, it is practically paused
at the first instruction to be executed. So device_add at that point is
considered as hotplug with all actions already executed on hardware level
(interrupts sent, devices responsible for hotplug handling has changed state).
So if one wished to delete device at that point, one would have to rollback
related state changes.
If one would additionally use -incoming CLI option, it becomes more complicated
as we might endup in prelaunch runstate with VM in running state
(see possible transitions in runstate_transitions_def[])
I'd say prelauch runstate can't be used for removing devices that do not
support surprise removal (in our case PCI isn't).

> 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.
Regarding ACPI based PCI hotplug you can look at
  docs/specs/acpi_pci_hotplug.txt
  hw/acpi/pcihp.c
  ACPI AML part in build_append_pci_bus_devices()

> 
> Thanks.