Re: [PATCH v4 5/7] arm64/futex: add futex atomic operation with FEAT_LSUI

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Jul 21, 2025 at 09:36:16AM +0100, Yeoreum Yun wrote:
> 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?

> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> ---
>  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.

Mark.