Re: [Qemu-devel] [PATCH RFC] docs: add PCIe devices placement guidelines

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> > +Plug only legacy PCI devices as Root Complex Integrated Devices
> > +even if the PCIe spec does not forbid PCIe devices.
> 
> I suggest "even though the PCI Express spec does not forbid PCI Express
> devices as Integrated Devices". (Detail is good!)

While talking about integrated devices:  There is docs/q35-chipset.cfg,
which documents how to mimic q35 with integrated devices as close and
complete as possible.

Usage:
  qemu-system-x86_64 -M q35 -readconfig docs/q35-chipset.cfg $args

Side note for usb: In practice you don't want to use the tons of
uhci/ehci controllers present in the original q35 but plug xhci into one
of the pcie root ports instead (unless your guest doesn't support xhci).

> > +as required by PCI spec will reserve a 4K IO range for each.
> > +The firmware used by QEMU (SeaBIOS/OVMF) will further optimize
> > +it by allocation the IO space only if there is at least a device
> > +with IO BARs plugged into the bridge.
> 
> This used to be true, but is no longer true, for OVMF. And I think it's
> actually correct: we *should* keep the 4K IO reservation per PCI-PCI bridge.
> 
> (But, certainly no IO reservation for PCI Express root port, upstream
> port, or downstream port! And i'll need your help for telling these
> apart in OVMF.)

IIRC the same is true for seabios, it looks for the pcie capability and
skips io space allocation on pcie ports only.

Side note: the linux kernel allocates io space nevertheless, so
checking /proc/ioports after boot doesn't tell you what the firmware
did.

cheers,
  Gerd