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AJvYcCUXub4OoFXMLyHuocy2uCkdNLM+OA0ESFBYSEh8CKDuK9EpDsG4z0+dSL00WuBul5JxOVlN6jsw9kRhm2s=@vger.kernel.org X-Gm-Message-State: AOJu0YzUrX+Tu37i2kkU6UvALZP9b643kaQlrniO4+J6rer797pgdvrW OiGZylaMRu7afLaSY00bxFY9OK86JLWHyL2bMC70DuFL8S/zfMX3aVgCQpxiJaI+7nFRn50pgnB 3BeidkYR9rxJQFFv1dw== X-Received: from edvd12.prod.google.com ([2002:aa7:ce0c:0:b0:659:31c4:b9d0]) (user=aliceryhl job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6402:27d0:b0:65c:b71:e69e with SMTP id 4fb4d7f45d1cf-65e825a4ef5mr496694a12.8.1771575396787; Fri, 20 Feb 2026 00:16:36 -0800 (PST) Date: Fri, 20 Feb 2026 08:16:35 +0000 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260130-gpuvm-rust-v4-0-8364d104ff40@google.com> <20260130-gpuvm-rust-v4-3-8364d104ff40@google.com> Message-ID: Subject: Re: [PATCH v4 3/6] rust: gpuvm: add GpuVm::obtain() From: Alice Ryhl To: Danilo Krummrich Cc: Daniel Almeida , Boris Brezillon , Janne Grunau , Matthew Brost , "Thomas =?utf-8?Q?Hellstr=C3=B6m?=" , Lyude Paul , Asahi Lina , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org Content-Type: text/plain; charset="utf-8" On Thu, Feb 19, 2026 at 08:22:14PM +0100, Danilo Krummrich wrote: > On Fri Jan 30, 2026 at 3:24 PM CET, Alice Ryhl wrote: > > +/// Represents that a given GEM object has at least one mapping on this [`GpuVm`] instance. > > +/// > > +/// Does not assume that GEM lock is held. > > +#[repr(C)] > > +#[pin_data] > > +pub struct GpuVmBo { > > + #[pin] > > + inner: Opaque, > > + #[pin] > > + data: T::VmBoData, > > +} > > + > > +impl GpuVmBo { > > + pub(super) const ALLOC_FN: Option *mut bindings::drm_gpuvm_bo> = { > > + use core::alloc::Layout; > > + let base = Layout::new::(); > > + let rust = Layout::new::(); > > + assert!(base.size() <= rust.size()); > > + if base.size() != rust.size() || base.align() != rust.align() { > > + Some(Self::vm_bo_alloc) > > + } else { > > + // This causes GPUVM to allocate a `GpuVmBo` with `kzalloc(sizeof(drm_gpuvm_bo))`. > > + None > > So, if T::VmBoData is a ZST *and* needs drop, we may end up allocating on the C > side and freeing on the Rust side. > > I assume this is intentional and there is nothing wrong with it, but without a > comment it might be a bit subtle. Yeah it's subtle but correct. > Another subtlety is that vm_bo_free() and vm_bo_alloc() assume that inner is > always the first member. I'd probably add a brief comment why this even has to > be the case, i.e. vm_bo_alloc() does not return *mut c_void, but *mut > bindings::drm_gpuvm_bo. I will add comments. > > + /// Access this [`GpuVmBo`] from a raw pointer. > > + /// > > + /// # Safety > > + /// > > + /// For the duration of `'a`, the pointer must reference a valid `drm_gpuvm_bo` associated with > > + /// a [`GpuVm`]. > > + #[inline] > > + pub unsafe fn from_raw<'a>(ptr: *mut bindings::drm_gpuvm_bo) -> &'a Self { > > I think this a good candidate for crate private, as we don't want drivers to use > this, but still use it in other DRM core modules. > > > + // SAFETY: `drm_gpuvm_bo` is first field and `repr(C)`. > > + unsafe { &*ptr.cast() } > > + } > > + > > + /// Returns a raw pointer to underlying C value. > > + #[inline] > > + pub fn as_raw(&self) -> *mut bindings::drm_gpuvm_bo { > > Less important, but probably also only needed in core DRM code. For cases like these two, I do think one can run into cases where you want them to be public. E.g. the vma confusion bugfix uses a raw pointer for now: https://lore.kernel.org/all/20260218-binder-vma-check-v2-1-60f9d695a990@google.com/ I'm generally not so worried about methods like these being public because they can't be used without unsafe. > > + /// Look up whether there is an existing [`GpuVmBo`] for this gem object. > > + #[inline] > > + pub(super) fn obtain(self) -> GpuVmBoRegistered { > > + let me = ManuallyDrop::new(self); > > + // SAFETY: Valid `drm_gpuvm_bo` not already in the lists. > > + let ptr = unsafe { bindings::drm_gpuvm_bo_obtain_prealloc(me.as_raw()) }; > > + > > + // Add the vm_bo to the extobj list if it's an external object, and if the vm_bo does not > > + // already exist. (If we are using an existing vm_bo, it's already in the extobj list.) > > + if ptr::eq(ptr, me.as_raw()) && me.gpuvm().is_extobj(me.obj()) { > > + let resv_lock = me.gpuvm().raw_resv(); > > + // SAFETY: The GPUVM is still alive, so its resv lock is too. > > + unsafe { bindings::dma_resv_lock(resv_lock, ptr::null_mut()) }; > > Maybe add a TODO to replace this with a proper lock guard once available? Ok. > > +/// A [`GpuVmBo`] object in the GEM list. > > +/// > > +/// # Invariants > > +/// > > +/// Points at a `drm_gpuvm_bo` that contains a valid `T::VmBoData` and is present in the gem list. > > +pub struct GpuVmBoRegistered(NonNull>); > > I know that I proposed to rename this from GpuVmBoResident to GpuVmBoRegistered > in a drive-by comment on v3. > > But now that I have a closer look, I think it would be nice to just have GpuVmBo > being the registered one and GpuVmBoAlloc being the pre-allocated one. > > As it is currently, I think it is bad to ever present a &GpuVmBo to a driver > because it implies that we don't know whether it is a pre-allocated one or a > "normal", registered one. But we do always know. Actually, I think GpuVmBo is already the registered one. GpuVmBoRegistered is just ARef>. > For instance, in patch 6 we give out &'op GpuVmBo, but it actually carries > the invariant of being registered. > > Of course, we could fix this by giving out a &'op GpuVmBoRegistered instead, > but it would be nice to not have drivers be in touch with a type that can be one > or the other. > > I know that the current GpuVmBo also serves the purpose of storing common > code. Maybe we can make it private, call it GpuVmBoInner and have inline > forwarding methods for GpuVmBo and GpuVmBoAlloc. This is slightly more > overhead in this implementation due to the forwarding methods, but less > ambiguity for drivers, which I think is more important. I think we should keep the current state that GpuVmBo is registered, and only GpuVmBoAlloc is not. That is most useful. Alice