Messages in this thread |  | | Date | Mon, 1 Dec 2025 09:58:56 +0000 | | Subject | Re: [PATCH v3] io: add io_pgtable abstraction | | From | Alice Ryhl <> |
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On Fri, Nov 28, 2025 at 04:47:52PM +0000, Robin Murphy wrote: > On 2025-11-28 12:27 pm, Alice Ryhl wrote: > [...] > > > > + /// 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? > > As an assumption of use for the Rust API, like I say it's fine - it's still > not really "unsafe" if a caller did try an overlapping mapping; the call > will still fail gracefully and accurately, it's just it will also fire a > WARN_ON() since ARM_64_LPAE_S1 without IO_PGTABLE_QUIRK_NO_WARN considers > this indicative of a usage error or race in the caller. > > If we do end up wanting to support more opportunistic and/or > userspace-controlled mappings by Rust drivers in future then we can relax > this expectation as appropriate.
Yeah, let's just say that it's an unsupported use-case. These bindings can be expanded in the future if anyone needs QUICK_NO_WARN.
> > > > + /// * 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. > > Aha, thanks for clarifying - indeed this is not the common "value or error" > case, it is two (almost) orthogonal return values. However if we're not > permitting callers to try to do anything clever with -EEXIST then it might > make sense to just embed the inevitable cleanup-on-failure boilerplate here > anyway (even if we still leave retry-on-partial-success to the caller).
Is the only possible error -EEXIST? I could encode that in the API if that is the case.
> Note that it does appear to be the case that io-pgtable-arm in its current > state won't actually do this, since it happens to handle all its error > return cases before any leaf PTEs are touched and "mapped" is updated, but > the abstraction layer shouldn't assume that in general since other > implementations like io-pgtable-arm-v7s definitely *can* fail with a partial > mapping.
Agreed, I will update the API accordingly.
> > > > + } > > > > + > > > > + /// 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`. > > Yes, that's a nice way to put it.
Perfect thanks.
Alice
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