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charset=UTF-8 Date: Tue, 19 May 2026 16:56:44 +0900 Message-Id: Cc: , , , , , , , Subject: Re: [PATCH v3 17/27] rust: auxiliary: generalize Registration over ForLt From: "Eliot Courtney" To: "Danilo Krummrich" , , , , , , , , , , , , , , , , , , , , , , , , , X-Mailer: aerc 0.21.0-0-g5549850facc2 References: <20260517000149.3226762-1-dakr@kernel.org> <20260517000149.3226762-18-dakr@kernel.org> In-Reply-To: <20260517000149.3226762-18-dakr@kernel.org> X-ClientProxiedBy: TYCP286CA0113.JPNP286.PROD.OUTLOOK.COM (2603:1096:400:29c::10) To BL0PR12MB2353.namprd12.prod.outlook.com (2603:10b6:207:4c::31) Precedence: bulk X-Mailing-List: nova-gpu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BL0PR12MB2353:EE_|CYXPR12MB9428:EE_ X-MS-Office365-Filtering-Correlation-Id: 4ac30055-e64e-40f7-e598-08deb57c2e1f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|10070799003|366016|376014|1800799024|7416014|18002099003|22082099003|3023799003|4143699003|921020|56012099003|11063799003; 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ForLt's covariance guarantee makes this sound. > > Signed-off-by: Danilo Krummrich > --- > drivers/gpu/nova-core/driver.rs | 4 +- > rust/kernel/auxiliary.rs | 68 +++++++++++++++++---------- > samples/rust/rust_driver_auxiliary.rs | 8 ++-- > 3 files changed, 52 insertions(+), 28 deletions(-) > > diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driv= er.rs > index fe4eafe1ebf0..ab0d7f2b516c 100644 > --- a/drivers/gpu/nova-core/driver.rs > +++ b/drivers/gpu/nova-core/driver.rs > @@ -21,6 +21,7 @@ > }, > Arc, > }, > + types::ForLt, > }; > =20 > use crate::gpu::Gpu; > @@ -32,7 +33,8 @@ > pub(crate) struct NovaCore { > #[pin] > pub(crate) gpu: Gpu, > - _reg: Devres>, > + #[allow(clippy::type_complexity)] > + _reg: Devres>, > } > =20 > const BAR0_SIZE: usize =3D SZ_16M; > diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs > index 3f1e2966c03a..db3d56106ea2 100644 > --- a/rust/kernel/auxiliary.rs > +++ b/rust/kernel/auxiliary.rs > @@ -20,6 +20,7 @@ > }, > prelude::*, > types::{ > + ForLt, > ForeignOwnable, > Opaque, // > }, > @@ -271,12 +272,16 @@ pub fn parent(&self) -> &device::Device { > =20 > /// Returns a pinned reference to the registration data set by the r= egistering (parent) driver. > /// > - /// Returns [`EINVAL`] if `T` does not match the type used by the pa= rent driver when calling > + /// `F` is the [`ForLt`](trait@ForLt) encoding of the data type. The= returned > + /// reference has its lifetime shortened from `'static` to `&self`'s= borrow lifetime via > + /// [`ForLt::cast_ref`]. > + /// > + /// Returns [`EINVAL`] if `F` does not match the type used by the pa= rent driver when calling > /// [`Registration::new()`]. > /// > /// Returns [`ENOENT`] if no registration data has been set, e.g. wh= en the device was > /// registered by a C driver. > - pub fn registration_data(&self) -> Result> { > + pub fn registration_data(&self) -> Result>>= { > // SAFETY: By the type invariant, `self.as_raw()` is a valid `st= ruct auxiliary_device`. > let ptr =3D unsafe { (*self.as_raw()).registration_data_rust }; > if ptr.is_null() { > @@ -289,18 +294,23 @@ pub fn registration_data(&self) -> Resu= lt> { > =20 > // SAFETY: `ptr` is non-null and was set via `into_foreign()` in= `Registration::new()`; > // `RegistrationData` is `#[repr(C)]` with `type_id` at offset 0= , so reading a `TypeId` > - // at the start of the allocation is valid regardless of `T`. > + // at the start of the allocation is valid regardless of `F`. > let type_id =3D unsafe { ptr.cast::().read() }; > - if type_id !=3D TypeId::of::() { > + if type_id !=3D TypeId::of::>() { > return Err(EINVAL); > } This seems a little dangerous to me, since the caller can specify the ForLt type which means it can rewrite the lifetime of the ForLt encoded type to be whatever it wants. The runtime check here only looks at the typeid of the encoded type but can't check the mapping from forlt::of given lifetime to actual underlying type lifetime instantiation. For example, in the next patch on the sample aux driver you can do: ``` let data =3D adev.registration_data:: Data<'static>)>()?; let pdev: &'static pci::Device =3D data.parent; ``` and mint a 'static reference to a bound device, which seems unsound to me. So maybe this needs to store the typeid of F instead? so that it can store the actual mapping from forlt lifetime to underlying type lifetimes. Overall I feel it's unfortunate to have this be a runtime check though. What about requiring all device ids declared to be supported by an aux bus driver to have the same rust registration data type? Not sure how common it is to want to have the same aux bus driver support a very hetereogenous set of aux bus devices though. I think getting rid of the runtime check was discussed on the other patch series adding the rust sideband registration data on the C side, but if there's not much real use case for hetereogenous registration data types in a single aux bus driver then it might be worth the added safety. WDYT?