Hi Mark,

quoted

Current futex atomic operations are implemented with ll/sc instructions and
clearing PSTATE.PAN.

Since Armv9.6, FEAT_LSUI supplies not only load/store instructions but
also atomic operation for user memory access in kernel it doesn't need
to clear PSTATE.PAN bit anymore.

With theses instructions some of futex atomic operations don't need to
be implmented with ldxr/stlxr pair instead can be implmented with
one atomic operation supplied by FEAT_LSUI.

However, some of futex atomic operations still need to use ll/sc way
via ldtxr/stltxr supplied by FEAT_LSUI since there is no correspondant
atomic instruction or doesn't support word size operation
(i.e) eor, cas{mb}t But It's good to work without clearing PSTATE.PAN bit.

That's unfortunate; have we fed back to Arm's architecture folks that we
care about those cases?

I haven’t done so yet. If you don’t mind,
could you let me know the appropriate person to give the feedback to?

quoted

Signed-off-by: Yeoreum Yun <redacted>
---
 arch/arm64/include/asm/futex_lsui.h | 132 ++++++++++++++++++++++++++++
 1 file changed, 132 insertions(+)
 create mode 100644 arch/arm64/include/asm/futex_lsui.h

This logic is introduced unused, and TBH I don't think this needs to be
in a separate header.

I reckon it's be better to keep all of this in <asm/futex.h> and rework
the series to:

(1) Factor out the existing LL/SC logic into separate LL/SC helpers in
    <asm/futex.h>, with an __llsc_ prefix, called by the existing
    functions.

(2) Add the new __lsui_ futex operations to <asm/futex.h>, along with
    code to select between the __llsc_ and __lsui_ versions.

We split the regular atomics different becuase there are *many* generic
atomic operations, but I don't think it's worthwhile to split the futex
logic over several headers.

Maybe it's worth having <asm/lsui.h>, but for now I reckon it's best to
also fold that into <asm/futex.h>, and we can split it out later if we
need it for something else.

Thanks for your suggestion.
I’ll rework it while keeping this implementation.

Thanks!

--
Sincerely,
Yeoreum Yun