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Date: Tue, 3 Feb 2026 18:19:03 +0100 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Jan Beulich Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Stewart Hildebrand Subject: Re: [PATCH v3 5/6] x86/PCI: avoid re-evaluation of extended config space accessibility Message-ID: References: <1bcdd2dd-9b2b-4eb2-a569-e28b03c1e1fb@suse.com> <4913d9b3-03ec-443e-b908-a1531dca6699@suse.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4913d9b3-03ec-443e-b908-a1531dca6699@suse.com> X-ClientProxiedBy: MR2P264CA0168.FRAP264.PROD.OUTLOOK.COM (2603:10a6:501:1::31) To CH7PR03MB7860.namprd03.prod.outlook.com (2603:10b6:610:24e::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH7PR03MB7860:EE_|DS4PR03MB8374:EE_ X-MS-Office365-Filtering-Correlation-Id: 0c338bdc-e154-4374-daaf-08de63485649 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014; 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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.