Add documentation to clarify that if `.valid.unaligned = true` but `.impl.unaligned = false`, QEMU’s memory core will automatically split unaligned guest accesses into multiple aligned accesses. This helps devices avoid implementing their own unaligned logic, but can be problematic for devices with side-effect-heavy registers. Also note that setting `.valid.unaligned = false` together with `.impl.unaligned = true` is invalid, as it contradicts itself and will trigger an assertion.
Signed-off-by: CJ Chen <cjc...@igel.co.jp> Acked-by: Tomoyuki Hirose <hrstmyk8...@gmail.com> Suggested-by: Peter Maydell <peter.mayd...@linaro.org> --- docs/devel/memory.rst | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/docs/devel/memory.rst b/docs/devel/memory.rst index 57fb2aec76..71d7de7ae5 100644 --- a/docs/devel/memory.rst +++ b/docs/devel/memory.rst @@ -365,6 +365,24 @@ callbacks are called: - .impl.unaligned specifies that the *implementation* supports unaligned accesses; if false, unaligned accesses will be emulated by two aligned accesses. +- Additionally, if .valid.unaligned = true but .impl.unaligned = false, the + memory core will emulate each unaligned guest access by splitting it into + multiple aligned sub-accesses. This ensures that devices which only handle + aligned requests do not need to implement unaligned logic themselves. For + example, see xhci_cap_ops in hw/usb/hcd-xhci.c: it sets .valid.unaligned + = true so guests can do unaligned reads on the xHCI Capability Registers, + while keeping .impl.unaligned = false to rely on the core splitting logic. + However, if a device’s registers have side effects on read or write, this + extra splitting can introduce undesired behavior. Specifically, for devices + whose registers trigger state changes on each read/write, splitting an access + can lead to reading or writing bytes beyond the originally requested subrange + thereby triggering repeated or otherwise unintended register side effects. + In such cases, one should set .valid.unaligned = false to reject unaligned + accesses entirely. +- Conversely, if .valid.unaligned = false but .impl.unaligned = true, + that setting is considered invalid; it claims unaligned access is allowed + by the implementation yet disallowed for the device. QEMU enforces this with + an assertion to prevent contradictory usage. API Reference ------------- -- 2.25.1
