Re: PCI Hotplug ACPI device names only 3 characters long

[Marcello Sylverster Bauer <sylv@sylv.io>]

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
>> Marcello
> 
> 
> 1.  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

> 
>