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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?amlYNU5lL1BDZ2s1QmJubFJzUGhxaFVMdmVFZCtrUmtiODc4QWFxcndiU2Mv?= =?utf-8?B?Q0Z2dWFHQXQzZjRuZG5ER3Vyb29yVU4yT1NOc2tnUjNRZnhVWTJrejViTVo5?= =?utf-8?B?cFY3ekVJcUlXWVhuM1hucGxsbkk5T29iOXpwS0l1VVhnU29JYkFKVDlVQzl1?= =?utf-8?B?L2I3RFBqeThISU5Jc2NRR1FjaDA5R0ZPTlNPQkxiSDRqYThZM0VmU01ZcFBF?= =?utf-8?B?clFaSTVudVFIUEp3ZFRCQU43YVZkZEhFeWxkVnBSVERkMy9XcG9CT0NpTEFB?= =?utf-8?B?bk1hMkFkeDUwaDBseWxXMm1OSzkrNndLWE44Ykg5bDZsU05lMUZybUUwdGpk?= =?utf-8?B?eWxVNGtvN08rZHAvRzR3c01tRTI1SlZvU3llMXZvS1V1OU9RaythNHAwR2xD?= =?utf-8?B?cG1jelpmaDFVNDZkMVJZbTB6WGtJMXhCUWh4a1pkZSt1NGJHb1cvQ290WDla?= =?utf-8?B?VlgxSDBubG5FMiswOVcrZWV6allvSkhTY2VIRlR5bVdsUmUxdGtqcXZpR0E5?= =?utf-8?B?ZERCUnZKSi9FZjdHZFRvUFQwTHBlOGY5bTNYVUVuZEgyZXUrNWZQRmNQNDFR?= =?utf-8?B?ZS83S05vQmZ5YkhQYWNYK1JkcnlyWVNYUzRpdlhWcjVZcnVQa3JnRTY1aHd5?= =?utf-8?B?VmdxMDVaRzNOMXpOQWNNQy9lcGlEU0ZLMDhDTU1uSy9xcnEzL3dqZVhHcFk0?= =?utf-8?B?WEFPRCsycGhHc1BjSm1HNVFFd294L1luOW9udm10WHZETmZQSEFIVUUwc0lZ?= =?utf-8?B?MEZMbWd1cmE0d2J6RUhtd2lkRzdRd290bitpbDVBQnBiVHFMUXIxTkovbElq?= =?utf-8?B?ZlpWam5QY01LcFY2dUZJVDA4R3ZoYWRzUkFnakZwcEdVbW9KTVpvUVZmRG9Y?= =?utf-8?B?bm8xQ3o3TjJwWlV1SEtHQTA0Y0Fnbk02L0RyQ21vSDRXZW5qc0IrdUZoamtl?= =?utf-8?B?QnM0dVZZYXp3RVlYTTF4cVZmYm5ITm9JNytzTlB4VFMzNkdYa1lYUW1rQ0NQ?= =?utf-8?B?NkoyTUI3djZMSzlSTjhTdjlPQ0pvbzlPbUZQUXc0YkNYT1JNT3NTQ29WQVhp?= =?utf-8?B?dld1UC9XU3k3TTM0S3lQZ2tYRHNVaFVwTUd4YXhML3BWZXZmaGVOc3VuNTJD?= =?utf-8?B?NUYrR0JWZEZHNjZ4R2RwWmxuTTFiVmQ2L3ZzdFpmRHJsUHlScVVuV3lRaVJC?= =?utf-8?B?MnRzZXdnVzZ5NzBSNVlFdkJaMnpNS2Zpbk5HNXN6cFhVMWN2ZXd0bmRJMTVF?= =?utf-8?B?YkpySlZNTjhpbUlJMm1kdS9HZEZWdkFxTzVPSlFKVzRWTG1qNGg1MFJPMkxI?= =?utf-8?B?M25RQzZpNi9QTWFvdzNUQXQyVmpEaFp0MzJRd0pEejN4Y3R4aTNEOU5Ob25o?= =?utf-8?B?L3FSSEhKMW1GaE1ObzBPcjNuYStaNGhSWWdiUktHVmNkUXE5SkFZdzdsS0RS?= =?utf-8?B?cENTbitLRXVmdlFwc2FDbzB5NXEwYVowWlNFQVgydllZckpRaURmdmIrNFRS?= =?utf-8?B?RkZucGNsQTUrUk90Sm81T0FtODhJeG82c0craEFUVCtqSXBKR0FEeFJiRXAv?= =?utf-8?B?bDF1bnBJaUczZkhzN3E3eGl5VDBDdUZhUFBGcXlVbGdINGhEeUhHRGpLZmF5?= =?utf-8?B?bDhXSmpqNmtTMHcvbVhtalMva1hFdWVXQ0VnV3cvVElSOXBMYW9xWERjK1ZX?= =?utf-8?B?ZzdLTUhVUG9CZmxQNHg4c0ZjbUZnM1crM3ludndWbnJTSTRrOER3NlFZYWJF?= =?utf-8?Q?S+H4njhYesT/b+H0L1DeCnn+j/bgU1GAk9Fe/Y2?= X-MS-Exchange-CrossTenant-Network-Message-Id: 9b524273-2c64-47a8-5434-08d97770eec4 X-MS-Exchange-CrossTenant-AuthSource: DS7PR03MB5608.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Sep 2021 11:15:17.6970 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 335836de-42ef-43a2-b145-348c2ee9ca5b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: buX4q2KgBa+ck5wZZv/i7p959I2i8O+Ne6iDNA7jBjmOG9yB+4WRWHBpxjJZBzDMBdLic0GddyhZRG5X79AS4A== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR03MB2922 X-OriginatorOrg: citrix.com Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 14, 2021 at 11:03:23AM +0200, Jan Beulich wrote: > On 14.09.2021 10:32, Roger Pau Monné wrote: > > On Tue, Sep 07, 2021 at 12:04:34PM +0200, Jan Beulich wrote: > >> In order to try to debug hypervisor side breakage from XSA-378 I found > >> myself urged to finally give PVH Dom0 a try. Sadly things didn't work > >> quite as expected. In the course of investigating these issues I actually > >> spotted one piece of PV Dom0 breakage as well, a fix for which is also > >> included here. > >> > >> There are two immediate remaining issues (also mentioned in affected > >> patches): > >> > >> 1) It is not clear to me how PCI device reporting is to work. PV Dom0 > >> reports devices as they're discovered, including ones the hypervisor > >> may not have been able to discover itself (ones on segments other > >> than 0 or hotplugged ones). The respective hypercall, however, is > >> inaccessible to PVH Dom0. Depending on the answer to this, either > >> the hypervisor will need changing (to permit the call) or patch 2 > >> here will need further refinement. > > > > I would rather prefer if we could limit the hypercall usage to only > > report hotplugged segments to Xen. Then Xen would have to scan the > > segment when reported and add any devices found. > > > > Such hypercall must be used before dom0 tries to access any device, as > > otherwise the BARs won't be mapped in the second stage translation and > > the traps for the MCFG area won't be setup either. > > This might work if hotplugging would only ever be of segments, and not > of individual devices. Yet the latter is, I think, a common case (as > far as hotplugging itself is "common"). Right, I agree to use hypercalls to report either hotplugged segments or devices. However I would like to avoid mandating usage of the hypercall for non-hotplug stuff, as then OSes not having hotplug support don't really need to care about making use of those hypercalls. > Also don't forget about SR-IOV VFs - they would typically not be there > when booting. They would materialize when the PF driver initializes > the device. This is, I think, something that can be dealt with by > intercepting writes to the SR-IOV capability. My plan was to indeed trap SR-IOV capability accesses, see: https://lore.kernel.org/xen-devel/20180717094830.54806-1-roger.pau@citrix.com/ I just don't have time ATM to continue this work. > But I wonder whether > there might be other cases where devices become "visible" only while > the Dom0 kernel is already running. I would consider those kind of hotplug devices, and hence would require the use of the hypercall in order to notify Xen about them. > >> 2) Dom0, unlike in the PV case, cannot access the screen (to use as a > >> console) when in a non-default mode (i.e. not 80x25 text), as the > >> necessary information (in particular about VESA-bases LFB modes) is > >> not communicated. On the hypervisor side this looks like deliberate > >> behavior, but it is unclear to me what the intentions were towards > >> an alternative model. (X may be able to access the screen depending > >> on whether it has a suitable driver besides the presently unusable > >> /dev/fb based one.) > > > > I had to admit most of my boxes are headless servers, albeit I have > > one NUC I can use to test gfx stuff, so I don't really use gfx output > > with Xen. > > > > As I understand such information is fetched from the BIOS and passed > > into Xen, which should then hand it over to the dom0 kernel? > > That's how PV Dom0 learns of the information, yes. See > fill_console_start_info(). (I'm in the process of eliminating the > need for some of the "fetch from BIOS" in Xen right now, but that's > not going to get us as far as being able to delete that code, no > matter how much in particular Andrew would like that to happen.) > > > I guess the only way for Linux dom0 kernel to fetch that information > > would be to emulate the BIOS or drop into realmode and issue the BIOS > > calls? > > Native Linux gets this information passed from the boot loader, I think > (except in the EFI case, as per below). > > > Is that an issue on UEFI also, or there dom0 can fetch the framebuffer > > info using the PV EFI interface? > > There it's EFI boot services functions which can be invoked before > leaving boot services (in the native case). Aiui the PVH entry point > lives logically past any EFI boot services interaction, and hence > using them is not an option (if there was EFI firmware present in Dom0 > in the first place, which I consider difficult all by itself - this > can't be the physical system's firmware, but I also don't see where > virtual firmware would be taken from). > > There is no PV EFI interface to obtain video information. With the > needed information getting passed via start_info, PV has no need for > such, and I would be hesitant to add a fundamentally redundant > interface for PVH. The more that the information needed isn't EFI- > specific at all. I think our only option is to expand the HVM start info information to convey that data from Xen into dom0. Thanks, Roger.