[PATCH v5 0/4] rust: byte-wise atomic copies between `Page` and shared memory

[Andreas Hindborg <a.hindborg@kernel.org>]

This series adds a way to copy data between a `Page` and memory that is
shared with actors outside the kernel, such as user space threads or
hardware, without introducing a data race.

The copy is built on `SysMem` from `kernel::io`, which already accesses
system memory with volatile or byte-wise atomic operations. The series
first makes the requirements of `SysMem` explicit by requiring freedom
from plain accesses for the lifetime of a view. It then adds
`atomic_per_byte_memcpy()`, a wrapper around the kernel's `memcpy()`
with explicit safety requirements, and routes the `SysMem` copies
through it. Finally, it adds `Page::{read,write}_bytewise_atomic`, which
take a `SysMem<'_, [u8]>` view of the shared buffer.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
Changes in v5:
- Rebase on v7.4-rc2.
- Build on `SysMem` from `kernel::io` instead of adding raw pointer
  based helpers to `Page` (Gary).
- Add a patch that requires freedom from plain accesses for `SysMem`
  views (Gary).
- Route the `SysMemBackend` copies through `atomic_per_byte_memcpy()`,
  so that the requirements of the `memcpy()` call are explicit.
- Let `Page::{read,write}_bytewise_atomic` take a `SysMem<'_, [u8]>`
  instead of a raw pointer and a length.
- Drop the claim that `atomic_per_byte_memcpy()` is volatile.
- Add a non-overlap requirement to `atomic_per_byte_memcpy()`.
- Add examples to `Page::{read,write}_bytewise_atomic` (Alice).
- Link to v4: https://msgid.link/20260605-page-volatile-io-v4-0-0e217d9bf11a@kernel.org

Changes in v4:
- Split `atomic_per_byte_memcpy` out into its own patch (Boqun).
- Expand `atomic_per_byte_memcpy` documentation to explain the
  concurrent-safe semantics and to note that the kernel's `memcpy()`
  uses byte-wise atomic load/store instructions (Boqun).
- Clarify the src/dst asymmetry in the documentation of
  `Page::{read,write}_bytewise_atomic` (Boqun).
- Use the "valid for atomic {reads,writes}" shorthand in the safety
  lists of the new methods and of `atomic_per_byte_memcpy` (Benno).
- Drop the redundant data-race bullets that v3 added on
  `Page::{read,write}_raw` (Miguel).
- Link to v3: https://msgid.link/20260213-page-volatile-io-v3-1-d60487b04d40@kernel.org

Changes in v3:
- Update documentation adn safety requirements for `Page::{read,write}_bytewise_atomic`.
- Update safety comments in `Page::{read,write}_bytewise_atomic`.
- Call the correct copy function in `Page::{read,write}_bytewise_atomic`.
- Link to v2: https://msgid.link/20260212-page-volatile-io-v2-1-a36cb97d15c2@kernel.org

Changes in v2:
- Rewrite patch with byte-wise atomic operations as foundation of operation.
- Update subject and commit message.
- Link to v1: https://lore.kernel.org/r/20260130-page-volatile-io-v1-1-19f3d3e8f265@kernel.org

---
Andreas Hindborg (4):
      rust: io: require freedom from plain accesses for `SysMem`
      rust: sync: atomic: add atomic_per_byte_memcpy
      rust: io: copy `SysMem` with `atomic_per_byte_memcpy`
      rust: page: add byte-wise atomic copy methods for `SysMem`

 rust/kernel/io.rs          |  72 +++++++++++++++++----------
 rust/kernel/page.rs        | 121 +++++++++++++++++++++++++++++++++++++++++++++
 rust/kernel/sync/atomic.rs |  35 +++++++++++++
 3 files changed, 203 insertions(+), 25 deletions(-)
---
base-commit: 7475ea8b58616b33c04afb976cf0fbaad9dd63fb
change-id: 20260130-page-volatile-io-05ff595507d3

Best regards,
--  
Andreas Hindborg <a.hindborg@kernel.org>