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Thu, 28 Oct 2021 02:07:46 +0000 From: "Tian, Kevin" To: Jason Gunthorpe CC: Alex Williamson , "Liu, Yi L" , "hch@lst.de" , "jasowang@redhat.com" , "joro@8bytes.org" , "jean-philippe@linaro.org" , "parav@mellanox.com" , "lkml@metux.net" , "pbonzini@redhat.com" , "lushenming@huawei.com" , "eric.auger@redhat.com" , "corbet@lwn.net" , "Raj, Ashok" , "yi.l.liu@linux.intel.com" , "Tian, Jun J" , "Wu, Hao" , "Jiang, Dave" , "jacob.jun.pan@linux.intel.com" , "kwankhede@nvidia.com" , "robin.murphy@arm.com" , "kvm@vger.kernel.org" , "iommu@lists.linux-foundation.org" , "dwmw2@infradead.org" , "linux-kernel@vger.kernel.org" , "baolu.lu@linux.intel.com" , "david@gibson.dropbear.id.au" , "nicolinc@nvidia.com" Subject: RE: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO Thread-Topic: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO Thread-Index: AQHXrSGNbNtRgavabUSKJjvt8l12BauwlhaAgAAouwCAACufAIAAEL4QgACKr4CACtdfoIAA3DaAgBUX8GCAAHivgIAAl4FggAl6ZNCAAXETAIAAHLdQgAYt1oCAA0rdUA== Date: Thu, 28 Oct 2021 02:07:46 +0000 Message-ID: References: <20210923114219.GG964074@nvidia.com> <20210930222355.GH964074@nvidia.com> <20211014154259.GT2744544@nvidia.com> <20211021233036.GN2744544@nvidia.com> <20211025233459.GM2744544@nvidia.com> In-Reply-To: <20211025233459.GM2744544@nvidia.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: dlp-version: 11.6.200.16 dlp-product: dlpe-windows dlp-reaction: no-action authentication-results: nvidia.com; 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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: BN9PR11MB5433.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: e1b8675b-9cf1-4667-615f-08d999b7bc11 X-MS-Exchange-CrossTenant-originalarrivaltime: 28 Oct 2021 02:07:46.0407 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: ACeEfeGiDUy4o6UmIbFfByDaW71eo6jyvNxQLbtwVRupXuwFvDB1uZQjIPMfGpke24keXNnjI1kuUw8XusdfLw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN9PR11MB5420 X-OriginatorOrg: intel.com Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > From: Jason Gunthorpe > Sent: Tuesday, October 26, 2021 7:35 AM >=20 > On Fri, Oct 22, 2021 at 03:08:06AM +0000, Tian, Kevin wrote: >=20 > > > I have no idea what security model makes sense for wbinvd, that is th= e > > > major question you have to answer. > > > > wbinvd flushes the entire cache in local cpu. It's more a performance > > isolation problem but nothing can prevent it once the user is allowed > > to call this ioctl. This is the main reason why wbinvd is a privileged > > instruction and is emulated by kvm as a nop unless an assigned device > > has no-snoop requirement. alternatively the user may call clflush > > which is unprivileged and can invalidate a specific cache line, though > > not efficient for flushing a big buffer. > > > > One tricky thing is that the process might be scheduled to different > > cpus between writing buffers and calling wbinvd ioctl. Since wbvind > > only has local behavior, it requires the ioctl to call wbinvd on all > > cpus that this process has previously been scheduled on. >=20 > That is such a hassle, you may want to re-open this with the kvm > people as it seems ARM also has different behavior between VM and > process here. >=20 > The ideal is already not being met, so maybe we can keep special > casing cache ops? >=20 Now Paolo confirmed wbinvd ioctl is just a thought experiment.=20 Then Jason, want to have a clarification on 'keep special casing' here. Did you mean adopting the vfio model which neither allows the user to decide no-snoop format nor provides a wbinvd ioctl for the user to manage buffers used for no-snoop traffic, or still wanting the user=20 to decide no-snoop format but not implementing a wbinvd ioctl? The latter option sounds a bit incomplete from uAPI p.o.v. but it allows us to stay with one-format-one-ioas policy. And anyway the userspace can still call clflush to do cacheline-based invalidation, if necessary. The former option would force us to support multi-formats-one-ioas. either case it's iommufd which decides and tells kvm whether wbinvd=20 is allowed for a process.=20 Thanks Kevin