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From: | Marcello Sylverster Bauer |
Subject: | Re: PCI Hotplug ACPI device names only 3 characters long |
Date: | Tue, 5 Sep 2023 19:45:12 +0200 |
Hi Michael, On 9/5/23 18:44, Michael S. Tsirkin wrote:
On Tue, Sep 05, 2023 at 05:05:33PM +0200, Marcello Sylverster Bauer wrote:Greetings, I'm currently working on a project to support Intel IPU6 in QEMU via VFIO so that the guest system can access the camera. This requires extending the ACPI device definition so that the guest knows how to access the camera. However, I cannot extend the PCI devices because their names are not 4 characters long and therefore do not follow the ACPI specification. When I use '-acpitable' to include my own SSDT for the IPU6 PCI device, it does not allow me to declare the device as an External Object because it automatically adds padding underscores. e.g. Before: ``` External(_SB.PCI0.S18.SA0, DeviceObj) ``` After: ``` External(_SB.PCI0.S18_.SA0_, DeviceObj) ``` Adding the underscore padding is hard coded in iASL and also in QEMU when parsing an ASL file. (see: build_append_nameseg()) So here are my questions: 1. Is there a solution to extend the ACPI PCI device using '-acpitable' without having to patch iASL or QEMU? 2. Are there any plans to change the names to comply with the ACPI spec? (e.g. use "S%.03X" format string instead) Thanks Marcello1. All names in ACPI are always exactly 4 characters long. _ is a legal character but names beginning with _ are reserved.
Exactly, which is why I want to address this issue here. Currently, Qemu generates ACPI device names with only 3 characters. (See build_append_pci_bus_devices() in hw/i386/acpi-build.c). For example, the device I want to append entries to has the path "_SB.PCI0.S18.SA0", but I can't because of the two auto-generated devices with only 3 characters in their names.
There's no rule in ACPI spec that says they need to follow S%.03X or any other specific format. I'm pretty sure we do follow the ACPI specification in this but feel free to prove me wrong.
You have misunderstood me. Currently, Qemu uses the following format to create PCI ACPI devices:
``` aml_name("S%.02X", devfn) ```My question is whether we should change it to something that results in a 4 character name like "S%.03X" or "S%.02X_".
I have tested it and it works fine as long as any hardcoded path references are adjusted. But I'm not 100% sure if this could cause any regressions.
2. You can probably add something to existing ACPI devices using Scope().
I'm pretty sure the external object is required when loading a separate SSDT, but I'll try by just using scopes.
I would not advise relying on this - current names are not a stable interface that we guarantee across QEMU versions. If adding this functionality is desirable, I think we'll need some new interface to set a stable ACPI name. Maybe using aliases.
Currently I'm just working on a PoW to get IPU6 working in QEMU, so instability is fine.
Thanks, Marcello
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