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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Feb 03, 2026 at 03:48:52PM +0100, Jan Beulich wrote:
> 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.

TBH this is what the spec says, but I've never encountered such a
system.  In fact I've never tested hotplug of a PCI host bridge.

Not sure this can be simulated with QEMU so that we could at least
test whatever fixes we plan to do in that area?  I guess we could
"fake" a bodge where Xen ignores the MCFG completely and only becomes
aware of the ECAM areas from what the hardware domain reports back.

Thanks, Roger.