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Mon, 27 Sep 2021 13:00:08 +0000 From: "Tian, Kevin" To: Jason Gunthorpe CC: "Liu, Yi L" , "alex.williamson@redhat.com" , "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 06/20] iommu: Add iommu_device_init[exit]_user_dma interfaces Thread-Topic: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma interfaces Thread-Index: AQHXrSF9WDj+Z+DU+UqMJ7tQK2EdmauuvK2AgACKnbCAALw6gIAHYVYAgABthQCAABACQA== Date: Mon, 27 Sep 2021 13:00:08 +0000 Message-ID: References: <20210919063848.1476776-1-yi.l.liu@intel.com> <20210919063848.1476776-7-yi.l.liu@intel.com> <20210921170943.GS327412@nvidia.com> <20210922123931.GI327412@nvidia.com> <20210927115342.GW964074@nvidia.com> In-Reply-To: <20210927115342.GW964074@nvidia.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: 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: c7534eb1-78c4-4783-0331-08d981b6bb98 X-MS-Exchange-CrossTenant-originalarrivaltime: 27 Sep 2021 13:00:08.0311 (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: taExu+HyJHe0KQatxWih4F84vlv4gAv4W1ZW/BDs7ulO8sMI6PkH/vRD6rTj5ShOqVnf4C2irUJu9mQWx3bJuw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN6PR1101MB2084 X-OriginatorOrg: intel.com Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > From: Jason Gunthorpe > Sent: Monday, September 27, 2021 7:54 PM >=20 > On Mon, Sep 27, 2021 at 09:42:58AM +0000, Tian, Kevin wrote: > > > From: Jason Gunthorpe > > > Sent: Wednesday, September 22, 2021 8:40 PM > > > > > > > > Ie the basic flow would see the driver core doing some: > > > > > > > > Just double confirm. Is there concern on having the driver core to > > > > call iommu functions? > > > > > > It is always an interesting question, but I'd say iommu is > > > foundantional to Linux and if it needs driver core help it shouldn't > > > be any different from PM, pinctl, or other subsystems that have > > > inserted themselves into the driver core. > > > > > > Something kind of like the below. > > > > > > If I recall, once it is done like this then the entire iommu notifier > > > infrastructure can be ripped out which is a lot of code. > > > > Currently vfio is the only user of this notifier mechanism. Now > > three events are handled in vfio_iommu_group_notifier(): > > > > NOTIFY_ADD_DEVICE: this is basically for some sanity check. suppose > > not required once we handle it cleanly in the iommu/driver core. > > > > NOTIFY_BOUND_DRIVER: the BUG_ON() logic to be fixed by this change. > > > > NOTIFY_UNBOUND_DRIVER: still needs some thoughts. Based on > > the comments the group->unbound_list is used to avoid breaking >=20 > I have a patch series to delete the unbound_list, the scenario you > describe is handled by the device_lock() that's great! >=20 > > diff --git a/drivers/base/dd.c b/drivers/base/dd.c > > index 68ea1f9..826a651 100644 > > +++ b/drivers/base/dd.c > > @@ -566,6 +566,10 @@ static int really_probe(struct device *dev, struct > device_driver *drv) > > goto done; > > } > > > > + ret =3D iommu_device_set_dma_hint(dev, drv->dma_hint); > > + if (ret) > > + return ret; >=20 > I think for such a narrow usage you should not change the struct > device_driver. Just have pci_stub call a function to flip back to user > mode. Here we want to ensure that kernel dma should be blocked if the group is already marked for user-dma. If we just blindly do it for any driver at this point (as you commented earlier): + ret =3D iommu_set_kernel_ownership(dev); + if (ret) + return ret; how would pci-stub reach its function to indicate that it doesn't=20 do dma and flip back? Do you envision a simpler policy that no driver can be bound to the group if it's already set for user-dma? what about vfio-pci itself? >=20 > > +static int iommu_dev_viable(struct device *dev, void *data) > > +{ > > + enum dma_hint hint =3D *data; > > + struct device_driver *drv =3D READ_ONCE(dev->driver); >=20 > Especially since this isn't locked properly or safe. I have the same worry when copying from vfio. Not sure how vfio gets safe with this approach... Thanks Kevin