Re: [PATCH 3/5] hw/isa: Embed TYPE_PORT92 in south bridges used in PC machines

[BALATON Zoltan]

On Tue, 27 Feb 2024, Bernhard Beschow wrote:
> Am 21. Februar 2024 11:53:21 UTC schrieb Mark Cave-Ayland 
> <mark.cave-ayland@ilande.co.uk>:
> > On 18/02/2024 13:16, Bernhard Beschow wrote:
> > > Port 92 is an integral part of the PIIX and ICH south bridges, so instantiate it
> > > there. The isapc machine now needs to instantiate it explicitly, analoguous to
> > > the RTC.
> > >
> > > Note that due to migration compatibility, port92 is optional in the south
> > > bridges. It is always instantiated the isapc machine for simplicity.
> > >
> > > Signed-off-by: Bernhard Beschow <shentey@gmail.com>
> > > ---
> > >   include/hw/i386/pc.h          |  2 +-
> > >   include/hw/southbridge/ich9.h |  4 ++++
> > >   include/hw/southbridge/piix.h |  3 +++
> > >   hw/i386/pc.c                  | 18 ++++++++++++------
> > >   hw/i386/pc_piix.c             |  9 +++++++--
> > >   hw/i386/pc_q35.c              |  8 +++++---
> > >   hw/isa/lpc_ich9.c             |  9 +++++++++
> > >   hw/isa/piix.c                 |  9 +++++++++
> > >   hw/isa/Kconfig                |  2 ++
> > >   9 files changed, 52 insertions(+), 12 deletions(-)
> >
> > I had a look at this (and did a bit of revision around 8042 and A20), 
> > and I am starting to wonder if the PORT92 device isn't something that 
> > belongs to the southbridge, but more specifically to the superio chip?
>
> If there is agreement to model real hardware in QEMU, then I think that

I think there's no such agreement and QEMU is more lax about it both for 
historical reasons and to simplify machine models. Indeed, QEMU sometimes 
models non-existing machines (e.g. the mac99 or virt boards) that don't 
correspond to real hardware but allow guest OSes to boot. Even when 
modelllng real hardware it's ofren modelled just enough for guests to work 
and unused details are omitted for simplicity. It is recommended to follow 
what real hardware does when modelling real hardware but not always 
required. Although it might help both with verifying a device model and to 
compose machines with these models to try to follow the real hardware.

> port 92 belongs into any device model where the hardware has one. All 
> our PC-like southbridges (PIIX, ICH, VIA) have port 92. Many FDC37xxxx 
> including the FDC37M81x as used in the Malta board have one, too -- 
> where it must first be enabled.

So port92 is not a real hardware but a QEMU abstraction or model of some 
functionality found in some machines. Real chips probably implement this 
in different ways so we could either model this in these chips 
independently the same way as real hardware does or use the abstracted 
model anywhere in our machine model. Since this does not exist in real 
hardware as this abstract model it also does not belong anywhere so we are 
free to put it where it's most convenient or simple to do.

> > A couple of thoughts as to why I came to this conclusion: firstly the 
> > superio chip is generally considered to be a single integrated 
> > implementation of legacy IO devices, so this feels like a natural home 
> > for the PORT92 device.
>
> > Secondly the value of the "has-port92" property is controlled by 
> > pcms->i8042_enabled, and this value is already passed into functions 
> > such as pc_superio_init() for example.
>
> Rhight. There, it also controls the presence of port 92. If we move port 
> 92 into the southbridges, we have to respect this command line switch 
> there to preserve backward compatibility.
>
> I wonder what `-M i8042` is supposed to do. If it is for modeling a 
> stripped-down x86 system, why not use the microvm instead? How is it 
> possible to omit an essential piece of hardware needed to boot x86 
> systems? Don't we need at least either one (i8042 or port 92)?

Try git log -p 4ccd5fe22fe (found it via git blame and see what added that 
property).

> > I think this would also help reduce the changes required for the 
> > individual machines, however the devil is always in the details 
> > particularly when migration is involved.
>
> As stated above, this series is more about modeling real hardware, in 
> the hope that this will lend itself for configuration-driven machine 
> creation. It is also about identifying obstacles towards this goal. Does 
> it make sense to deprecate some machine-specific options such as i8042?

Only if you want to break downsteam users of those options but maybe they 
won't be happy about that.

Regards,
BALATON Zoltan