Re: [PATCH v3 5/6] x86/PCI: avoid re-evaluation of extended config space accessibility

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.02.2026 16:18, Roger Pau Monné wrote:
> My copy of the PCI Firmware Spec v3.3 contains:
> 
> "4.1.2. MCFG Table Description
> 
> The MCFG table is an ACPI table that is used to communicate the base
> addresses corresponding to the non-hot removable PCI Segment Groups
> range within a PCI Segment Group available to the operating system at
> boot.
> 
> [...]
> 
> 4.1.3. The _CBA Method
> 
> Some systems may support hot plug of host bridges that introduce
> either a range of buses within an existing PCI Segment Group or
> introduce a new PCI Segment Group. For example, each I/O chip in a
> multi-chip PCI Express root complex implementation could start a new
> PCI Segment Group."
> 
> Together with this:
> 
> "The MCFG table format allows for more than one memory mapped base
> address entry provided each entry (memory mapped configuration space
> base address allocation structure) corresponds to a unique PCI Segment
> Group consisting of 256 PCI buses. Multiple entries corresponding to a
> single PCI Segment Group is not allowed."
> 
> Given that each segment group can only appear once in the MCFG, and
> that the _CBA method can introduce new segment groups, it would seem
> to me the spec does allow for new segments appearing exclusively as
> the return of _CBA method?  It does read as if hot-removable segment
> groups must not appear in the MCFG table.  I'm not finding any clear
> statement in the spec that says that ECAM areas must previously appear
> in the MCFG table.
> 
> I'm not sure how common that is, but it doesn't seem impossible given
> my reading of the spec.

Hmm, that'll be a bit of work then, as Dom0 will also need to propagate
the necessary data into Xen.

Jan