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

[Michael S. Tsirkin]

On Wed, Oct 03, 2018 at 10:44:20AM -0700, open sorcerer wrote:
> Hi,
> 
> I am digging into an issue where qmp_device_del does not actually delete
> devices when a guest OS is in prelaunch.

What exactly is meant by prelaunch? E.g. is it prelaunch while bios is
doing the pci bus scan?

> 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?

Yes.

> 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.
> 

We are generally trying to move away from ACPI hotplug to native
PCIE hotplug. You can read up on that in the pci express spec.

-- 
MST