Re: PCI Hotplug ACPI device names only 3 characters long

[Michael S. Tsirkin]

On Tue, Sep 05, 2023 at 07:45:12PM +0200, Marcello Sylverster Bauer wrote:
> 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.

They are 4 characters otherwise OSPMs wouldn't work.
In your example the path is _SB.PCI0.S18_.SA0_ - you disassembler probably
just helpfully hides it for readability.

> > 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 think you misunderstand the code. Look at build_append_nameseg and you will
see that the name is always ACPI_NAMESEG_LEN characters which equals 4.

> 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
> 
> > 
> >