Messages in this thread |  | | Date | Fri, 28 Nov 2025 12:27:44 +0000 | | Subject | Re: [PATCH v3] io: add io_pgtable abstraction | | From | Alice Ryhl <> |
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On Fri, Nov 28, 2025 at 11:56:17AM +0000, Robin Murphy wrote: > On 2025-11-12 10:15 am, Alice Ryhl wrote: > > From: Asahi Lina <lina+kernel@asahilina.net> > > > > This will be used by the Tyr driver to create and modify the page table > > of each address space on the GPU. Each time a mapping gets created or > > removed by userspace, Tyr will call into GPUVM, which will figure out > > which calls to map_pages and unmap_pages are required to map the data in > > question in the page table so that the GPU may access those pages when > > using that address space. > > > > The Rust type wraps the struct using a raw pointer rather than the usual > > Opaque+ARef approach because Opaque+ARef requires the target type to be > > refcounted. > > > > Signed-off-by: Asahi Lina <lina+kernel@asahilina.net> > > Co-Developed-by: Alice Ryhl <aliceryhl@google.com> > > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > +/// Protection flags used with IOMMU mappings. > > +pub mod prot { > > + /// Read access. > > + pub const READ: u32 = bindings::IOMMU_READ; > > + /// Write access. > > + pub const WRITE: u32 = bindings::IOMMU_WRITE; > > + /// Request cache coherency. > > + pub const CACHE: u32 = bindings::IOMMU_CACHE; > > + /// Request no-execute permission. > > + pub const NOEXEC: u32 = bindings::IOMMU_NOEXEC; > > + /// MMIO peripheral mapping. > > + pub const MMIO: u32 = bindings::IOMMU_MMIO; > > + /// Privileged mapping. > > + pub const PRIV: u32 = bindings::IOMMU_PRIV; > > Nit: probably best to call this PRIVILEGED from day 1 for clarity - some day > we may eventually get round to renaming the C symbol too, especially if we > revisit the notion of "private" mappings (that's still on my ideas list...)
Sure, will rename.
> > + /// Map a physically contiguous range of pages of the same size. > > + /// > > + /// # Safety > > + /// > > + /// * This page table must not contain any mapping that overlaps with the mapping created by > > + /// this call. > > As mentioned this isn't necessarily true of io-pgtable itself, but since > you've not included QUIRK_NO_WARN in the abstraction then it's fair if this > layer wants to be a little stricter toward Rust users.
Assuming that we don't allow QUICK_NO_WARN, would you say that it's precise as-is?
> > + /// * If this page table is live, then the caller must ensure that it's okay to access the > > + /// physical address being mapped for the duration in which it is mapped. > > + #[inline] > > + pub unsafe fn map_pages( > > + &self, > > + iova: usize, > > + paddr: PhysAddr, > > + pgsize: usize, > > + pgcount: usize, > > + prot: u32, > > + flags: alloc::Flags, > > + ) -> Result<usize> { > > + let mut mapped: usize = 0; > > + > > + // SAFETY: The `map_pages` function in `io_pgtable_ops` is never null. > > + let map_pages = unsafe { (*self.raw_ops()).map_pages.unwrap_unchecked() }; > > + > > + // SAFETY: The safety requirements of this method are sufficient to call `map_pages`. > > + to_result(unsafe { > > + (map_pages)( > > + self.raw_ops(), > > + iova, > > + paddr, > > + pgsize, > > + pgcount, > > + prot as i32, > > + flags.as_raw(), > > + &mut mapped, > > + ) > > + })?; > > + > > + Ok(mapped) > > Just to double-check since I'm a bit unclear on the Rust semantics, this can > correctly reflect all 4 outcomes back to the caller, right? I.e.: > > - no error, mapped == pgcount * pgsize (success) > - no error, mapped < pgcount * pgsize (call again with the remainder) > - error, mapped > 0 (probably unmap that bit, unless clever trickery where > an error was expected) > - error, mapped == 0 (nothing was done, straightforward failure) > > (the only case not permitted is "no error, mapped == 0" - failure to make > any progress must always be an error) > > Alternatively you might want to consider encapsulating the partial-mapping > handling in this layer as well - in the C code that's done at the level of > the IOMMU API calls that io-pgtable-using IOMMU drivers are merely passing > through, hence why panfrost/panthor have to open-code their own equivalents, > but there's no particular reason to follow the *exact* same pattern here.
Ah, no this signature does not reflect all of those cases. The return type is Result<usize>, which corresponds to:
struct my_return_type { bool success; union { size_t ok; int err; // an errno } };
We need a different signature if it's possible to have mapped != 0 when returning an error.
> > + } > > + > > + /// Unmap a range of virtually contiguous pages of the same size. > > + /// > > + /// # Safety > > + /// > > + /// This page table must contain a mapping at `iova` that consists of exactly `pgcount` pages > > + /// of size `pgsize`. > > Again, the underlying requirement here is only that pgsize * pgcount > represents the IOVA range of one or more consecutive ranges previously > mapped, i.e.: > > map(0, 4KB * 256); > map(1MB, 4KB * 256); > unmap(0, 2MB * 1); > > is legal, since it's generally impractical for callers to know and keep > track of the *exact* structure of a given pagetable. In this case there > isn't really any good reason to try to be stricter.
How about this wording?
This page table must contain one or more consecutive mappings starting at `iova` whose total size is `pgcount*pgsize`.
Alice
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