Re: [PATCH for-6.2 00/43] Unaligned accesses for user-only

[Peter Maydell]

On Thu, 29 Jul 2021 at 01:50, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> This began with Peter wanting a cpu_ldst.h interface that can handle
> alignment info for Arm M-profile system mode, which will also compile
> for user-only without ifdefs.  This is patch 32.
>
> Once I had that interface, I thought I might as well enforce the
> requested alignment in user-only.  There are plenty of cases where
> we ought to have been doing that for quite a while.  This took rather
> more work than I imagined to start.
>
> So far only x86 host has been fully converted to handle unaligned
> operations in user-only mode.  I'll get to the others later.  But
> the added testcase is fairly broad, and caught lots of bugs and/or
> missing code between target/ and linux-user/.
>
> Notes:
>   * For target/i386 we have no way to signal SIGBUS from user-only.
>     In theory we could go through do_unaligned_access in system mode,
>     via #AC.  But we don't even implement that control in tcg, probably
>     because no one ever sets it.  The cmpxchg16b insn requires alignment,
>     but raises #GP, which maps to SIGSEGV.
>
>   * For target/s390x we have no way to signal SIGBUS from user-only.
>     The atomic operations raise PGM_SPECIFICATION, which the linux
>     kernel maps to SIGILL.
>
>   * I think target/hexagon should be setting TARGET_ALIGNED_ONLY=y.
>     In the meantime, all memory accesses are allowed to be unaligned.

Could you include suitable updates to docs/devel/loads-stores.rst
as well, please?

thanks
-- PMM