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

[Chuck Zmudzinski <brchuckz@xxxxxxx>]

On 1/4/2023 5:18 PM, Philippe Mathieu-Daudé wrote:
> 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.

Thanks for the nice explanation of the history. I understand the PIIX3 is 
associated
with the i440fx machine type for i386 - it goes all the way back to 1995 I think
with the original 32-bit Pentium processor and Windows 95. So it is a worthwhile
effort to work on updating this to something newer, and of course kvm can use
the newer Q35 chipset which goes back to 2009 or so, I think, but xen/x86 
languishes
on the i440fx for now.

Best regards,

Chuck