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Tue, 14 Jul 2026 15:29:23 +0000 Received: from BN7PR02MB4148.namprd02.prod.outlook.com ([fe80::6007:d1a1:bcf9:58ef]) by BN7PR02MB4148.namprd02.prod.outlook.com ([fe80::6007:d1a1:bcf9:58ef%6]) with mapi id 15.21.0223.008; Tue, 14 Jul 2026 15:29:23 +0000 From: Michael Kelley To: Yu Zhang , Michael Kelley CC: "linux-kernel@vger.kernel.org" , "linux-hyperv@vger.kernel.org" , "iommu@lists.linux.dev" , "linux-pci@vger.kernel.org" , "linux-arch@vger.kernel.org" , "wei.liu@kernel.org" , "kys@microsoft.com" , "haiyangz@microsoft.com" , "decui@microsoft.com" , "longli@microsoft.com" , "joro@8bytes.org" , "will@kernel.org" , "robin.murphy@arm.com" , "bhelgaas@google.com" , "kwilczynski@kernel.org" , "lpieralisi@kernel.org" , "mani@kernel.org" , "robh@kernel.org" , "arnd@arndb.de" , "jgg@ziepe.ca" , "jacob.pan@linux.microsoft.com" , "tgopinath@linux.microsoft.com" , "easwar.hariharan@linux.microsoft.com" , "mrathor@linux.microsoft.com" Subject: RE: [PATCH v2 3/4] iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest Thread-Topic: [PATCH v2 3/4] iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest Thread-Index: AQHdCjyk3FoDd2PvoEaR3JQ0We2DX7ZlmCnQgADQU4CAAknxcIADB1oAgAAOZJCAAKasgIAAx8IQ Date: Tue, 14 Jul 2026 15:29:23 +0000 Message-ID: References: <20260702160518.311234-1-zhangyu1@linux.microsoft.com> <20260702160518.311234-4-zhangyu1@linux.microsoft.com> <3ty6yq6oftsvq52skrngjv5xpyixhsyfo3dndhoujt7emxsb2o@y6ischifpmfn> In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-publictraffictype: Email x-ms-traffictypediagnostic: BN7PR02MB4148:EE_|CH3PR02MB9907:EE_ x-ms-office365-filtering-correlation-id: 04cacbac-eedf-4d2d-21e3-08dee1bcaee2 x-microsoft-antispam: BCL:0;ARA:14566002|13091999003|51005399006|8060799015|19110799012|8062599012|31061999003|15080799012|37011999003|24021099003|25010399006|40105399003|12091999003|102099032|3412199025|440099028|1710799026; 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charset="us-ascii" Content-Transfer-Encoding: quoted-printable Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-OriginatorOrg: outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: BN7PR02MB4148.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-CrossTenant-Network-Message-Id: 04cacbac-eedf-4d2d-21e3-08dee1bcaee2 X-MS-Exchange-CrossTenant-originalarrivaltime: 14 Jul 2026 15:29:23.6968 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa X-MS-Exchange-CrossTenant-rms-persistedconsumerorg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR02MB9907 From: Yu Zhang Sent: Monday, July 13, 2026 8= :34 PM >=20 > On Mon, Jul 13, 2026 at 05:37:51PM +0000, Michael Kelley wrote: > > From: Yu Zhang Sent: Monday, July 13, 20= 26 9:46 AM > > > > > > On Sat, Jul 11, 2026 at 06:31:15PM +0000, Michael Kelley wrote: > > > > From: Yu Zhang Sent: Friday, July 10= , 2026 12:34 AM > > > > [snip] > > > > > > > > > > One new thought: Have you considered the hibernate/resume > > > > cycle? Does anything need to be done with the pvIOMMU to > > > > make it functional again after resume? I see that the Intel and > > > > AMD IOMMU drivers have suspend and resume functions. I > > > > don't know enough about the Hyper-V pvIOMMU to know if it > > > > might also need suspend and resume functions. > > > > > > > > > > Thanks for raising this, Michael. We have not considered such support= . > > > > > > My understanding is that the Intel and AMD drivers only disable the > > > IOMMU translation, flush the IOTLB during the suspend and re-enable/ > > > reload the preserved root tables and other HW state during in the > > > resueme. > > > > > > But for pvIOMMU, I guess such job shall be done by the hypervisor? > > > For a device resumed on the same VM, its logical device ID should > > > also remain unchanged? And the corresponding Hyper-V domain objects, > > > configuration, and device attachments shall be preserved and restored > > > by hypervisor? I don't think the current Hyper-V ABI explicitly defin= es > > > this. But maybe if we want such feature, it could be done by the > > > hypervisor transparently? > > > > > > > I agree with your and Jacob's comments that the guest doesn't have > > any responsibility for saving/restoring IOMMU hardware state, as the > > Intel and AMD IOMMU drivers do. > > > > But yes, I'm wondering about the Hyper-V domain objects and device > > attachments. I doubt Hyper-V can do anything to save and restore > > them. Hibernation is a Linux concept that the Hyper-V host doesn't > > know anything about. > > > > Hibernation is already complicated, and in a VM it is even worse. :-( > > As a start, see Documentation/virt/hyperv/hibernation.rst, which I > > wrote about 18 months ago. It provides some basics as well as outlines > > the additional complexity in a Hyper-V guest VM. I'll also try to spend > > some time thinking through the implications for a pvIOMMU, and let > > you know if I have any more thoughts. > > >=20 > Thank you, Michael, and thanks for pointing us to the documentation. I > need some time to better understand the Linux guest hibernation and resum= e > flow and its implications for pvIOMMU. >=20 > Meanwhile, do you think this limitation should be documented in the > commit message or the cover letter? >=20 Yes, I'd say that the lack of hibernation/resume support should be noted in a commit message, probably with Patch 3 of the series. Michael