On Fri, Aug 22, 2025 at 06:24:02PM +0900, CJ Chen wrote: > 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.
Some cosmetic comments only.. This shouldn't be another bullet, but rather a separate sub-section, because it describes the relationship of above two entries. IMO it can be better like this: MMIO Operations --------------- ... - .valid.min_access_size, .valid.max_access_size... - .valid.unaligned... See :ref:`unaligned-mmio-accesses` for details. - .impl.min_access_size, .impl.max_access_size... - .impl.unaligned... See :ref:`unaligned-mmio-accesses` for details. .. _unaligned-mmio-accesses: Unaligned MMIO Accesses ~~~~~~~~~~~~~~~~~~~~~~~ ... -- Peter Xu
