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Date: Wed, 4 Feb 2026 18:46:56 +0100 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Andrew Cooper Cc: Jan Beulich , xen-devel@lists.xenproject.org, Anthony PERARD , Michal Orzel , Julien Grall , Stefano Stabellini Subject: Re: [PATCH v2] x86/domain: adjust limitation on shared_info allocation below 4G Message-ID: References: <20260204122553.75711-1-roger.pau@citrix.com> <764e6a9f-8def-4f6e-8311-ef91b6c3886e@citrix.com> <3c3d3309-956e-463d-ba03-2e557e79fbab@citrix.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3c3d3309-956e-463d-ba03-2e557e79fbab@citrix.com> X-ClientProxiedBy: MA3P292CA0002.ESPP292.PROD.OUTLOOK.COM (2603:10a6:250:2c::17) To CH7PR03MB7860.namprd03.prod.outlook.com (2603:10b6:610:24e::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH7PR03MB7860:EE_|SJ0PR03MB5678:EE_ X-MS-Office365-Filtering-Correlation-Id: 14300ba7-4a7b-4661-296c-08de6415659a X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024; 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> >>>>>> + /* Ensure all references to the old shared_info page are dropped. */ > >>>>>> + for_each_vcpu( d, v ) > >>>>>> + vcpu_info_reset(v); > >>>>> switch_compat() can only occur on a domain with no memory.  How can we > >>>>> have outstanding references? > >>>> As Jan pointed out, it's not references, but stashed pointers to the > >>>> previous shared_info page. I've used the wrong wording here. > >>> Yes, I saw that thread, but my question still stands. > >>> > >>> How can there be any this early in the domain's lifecycle? > >> Can't (aren't) vCPU-s added ahead of adding memory? > > At least on x86 when using xl/libxl the call to > > XEN_DOMCTL_set_address_size happens after the call to > > XEN_DOMCTL_max_vcpus, and the later calls vcpu_create() which sets the > > pointer into the shared_info page for legacy (< 32) vCPUs. > > Geez, that disaster of an interface still has spike traps we're falling > foul of. > > Please extend the comment to note the first 32 vcpu compatibility case. > > But, combined with the format change (clear vs copy page), doesn't it > mean there's an existing bug here? > > Even without moving the shared_info page, aren't the cached pointers > made wrong by switch_compat() ? No, they are not wrong because the vcpu_info array is at the start of shared_info, and it has the same size (64bytes) on both 32 and 64bits. Hence the offsets into the different vcpu_info array elements are the same regardless of bitness. > This is one area where we've got no XTF testing at all, and I probably > ought to see about fixing that. Yeah, it's not great. Thanks, Roger.