Re: [RFC V2 02/10] hw/pci: Move 'x-256b-flit' property from cxl_root_port to pcie_root_port.

[Junjie Cao <junjie.cao@intel.com>]

Hi Shrihari,

On Wed, 26 Aug 2026 11:04:02 +0530, Shrihari E S wrote:
> @@ -152,6 +152,7 @@ static const Property rp_props[] = {
> +    DEFINE_PROP_BOOL("x-256b-flit", PCIEPort, flitmode, true),
>  };

This moves the default=true along with the property, so it now applies
to every derived root port (pcie-root-port, ioh3420, pnv-phb-root-port,
aspeed.pcie-root-port), not just cxl-rp.  On an empty port the LNKSTA2
flit status bit flips at this patch alone -- pcie_cap_fill_slot_lnk()
consumes flitmode, and pcie_sync_bridge_lnk() only overwrites it once a
child is plugged -- so "does not impact any existing behavior" doesn't
cover the derived ports; with patch 6's pcie_cap_fill_lnk() additions
the ports also come up with Flit Mode Supported (Flags bit 15) and
LNKCTL bit 13 guest-writable.  I checked "-device pcie-root-port" and
"-device ioh3420" on q35 with the series applied: the Express Flags
read 0x8142 on both; before the series the property didn't exist on
these devices and bit 15 stayed 0.

That's a guest-visible config space change on non-CXL machines with no
compat handling.  Migration already breaks at this patch -- pc-q35-9.2
at both ends, plain "-device pcie-root-port", a build at the series
base on the source and one at 2/10 on the destination:

  get_pci_config_device: Bad config data: i=0x87 read: 0 device: 4 cmask: ff ...
  load of migration failed: ... '0000:00:03.0/pcie-root-port'

0x87 is the high byte of PCI_EXP_LNKSTA2; with the whole series on the
destination the first mismatch comes earlier, at 0x57, PCI_EXP_FLAGS.
Fresh q35 guests see different hardware too.  Was extending the default
beyond cxl-rp intended?  If yes, it needs hw_compat entries for older
machine types.  If not, defaulting to off here and setting flitmode
in a cxl-rp instance_init would preserve current behaviour for
non-CXL ports: a second DEFINE_PROP in a subclass asserts on the
duplicate name, and instance_init still runs before qdev applies
globals, so -global and hw_compat keep working.

Many thanks,
Junjie