Re: [PATCH v2 6/6] hw/isa/piix: Resolve redundant TYPE_PIIX3_XEN_DEVICE

[Philippe Mathieu-Daudé]

On 4/1/23 20:29, Chuck Zmudzinski wrote:
> On 1/4/23 1:48 PM, Philippe Mathieu-Daudé wrote:

> > Here TYPE_PIIX3_DEVICE means for "PCI function part of the PIIX
> > south bridge chipset, which expose a PCI-to-ISA bridge". A better
> > name could be TYPE_PIIX3_ISA_PCI_DEVICE. Unfortunately this
> > device is named "PIIX3" with no indication of ISA bridge.
>
>
> Thanks, you are right, I see the PIIX3 device still exists after
> this patch set is applied.
>
> chuckz@debian:~/sources-sid/qemu/qemu-7.50+dfsg/hw/i386$ grep -r PIIX3 *
> pc_piix.c:        pci_dev = pci_new_multifunction(-1, true, TYPE_PIIX3_DEVICE);
>
> I also understand there is the PCI-to-ISA bridge at 00:01.0 on the PCI bus:
>
> chuckz@debian:~$ lspci
> 00:00.0 Host bridge: Intel Corporation 440FX - 82441FX PMC [Natoma] (rev 02)

All these entries ('PCI functions') ...:

> 00:01.0 ISA bridge: Intel Corporation 82371SB PIIX3 ISA [Natoma/Triton II]
> 00:01.1 IDE interface: Intel Corporation 82371SB PIIX3 IDE [Natoma/Triton II]
> 00:01.2 USB controller: Intel Corporation 82371SB PIIX3 USB [Natoma/Triton II] 
> (rev 01)
> 00:01.3 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 03)

... are part of the same *device*: the PIIX south bridge.

This device is enumerated as #1 on the PCI bus #0.
It currently exposes 4 functions: ISA/IDE/USB/ACPI.

> 00:02.0 Unassigned class [ff80]: XenSource, Inc. Xen Platform Device (rev 01)
> 00:03.0 VGA compatible controller: Device 1234:1111 (rev 02)
>
> I also see with this patch, there is a bridge that is a PIIX4 ACPI at 00:01.3.
> I get the exact same output from lspci without the patch series, so that gives
> me confidence it is working as designed.

Historically the PIIX3 and PIIX4 QEMU models have been written by
different people with different goals.

- PIIX3 comes from x86 machines and is important for KVM/Xen
  accelerators
- PIIX4 was developed by hobbyist for MIPS machines

PIIX4 added the ACPI function which was proven helpful for x86 machines.

OS such Linux don't consider the PIIX south bridge as a whole chipset,
and enumerate each PCI function individually. So it was possible to add
the PIIX4 ACPI function to a PIIX3... A config that doesn't exist with
real hardware :/
While QEMU aims at modeling real HW, this config is still very useful
for KVM/Xen. So this Frankenstein config is accepted / maintained.

Bernhard is doing an incredible work merging the PIIX3/PIIX4 differences
into a more maintainable model :)

Regards,

Phil.