Re: [PATCH v11 RESEND 6/9] arm64: futex: support futex with FEAT_LSUI

Yeoreum Yun
2026-01-19T22:19:01Z

Hi Will,

> On Sun, Dec 14, 2025 at 11:22:45AM +0000, 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 operation don't have matched
> > instructuion i.e) eor or cmpxchg with word size.
> > For those operation, uses cas{al}t to implement them.
> >
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> > ---
> > arch/arm64/include/asm/futex.h | 180 ++++++++++++++++++++++++++++++++-
> > 1 file changed, 178 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
> > index f8cb674bdb3f..6778ff7e1c0e 100644
> > --- a/arch/arm64/include/asm/futex.h
> > +++ b/arch/arm64/include/asm/futex.h
> > @@ -9,6 +9,8 @@
> > #include <linux/uaccess.h>
> > #include <linux/stringify.h>
> >
> > +#include <asm/alternative.h>
> > +#include <asm/alternative-macros.h>
> > #include <asm/errno.h>
> >
> > #define FUTEX_MAX_LOOPS 128 /* What's the largest number you can think of? */
> > @@ -86,11 +88,185 @@ __llsc_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
> > return ret;
> > }
> >
> > +#ifdef CONFIG_AS_HAS_LSUI
> > +
> > +/*
> > + * When the LSUI feature is present, the CPU also implements PAN, because
> > + * FEAT_PAN has been mandatory since Armv8.1. Therefore, there is no need to
> > + * call uaccess_ttbr0_enable()/uaccess_ttbr0_disable() around each LSUI
> > + * operation.
> > + */
>
> I'd prefer not to rely on these sorts of properties because:
>
> - CPU bugs happen all the time
> - Virtualisation and idreg overrides mean illegal feature combinations
> can show up
> - The architects sometimes change their mind
>
> So let's either drop the assumption that we have PAN if LSUI *or* actually
> test that someplace during feature initialisation.

Thanks for detail explain. I'll drop the my silly assumption and
call the uaccess_ttbr0_enable()/disable() then.

>
> > +
> > +#define __LSUI_PREAMBLE ".arch_extension lsui\n"
> > +
> > +#define LSUI_FUTEX_ATOMIC_OP(op, asm_op, mb) \
> > +static __always_inline int \
> > +__lsui_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval) \
> > +{ \
> > + int ret = 0; \
> > + int oldval; \
> > + \
> > + asm volatile("// __lsui_futex_atomic_" #op "\n" \
> > + __LSUI_PREAMBLE \
> > +"1: " #asm_op #mb " %w3, %w2, %1\n" \
>
> What's the point in separating the barrier suffix from the rest of the
> instruction mnemonic? All the callers use -AL.

Agree. I'll remove this.

>
> > +"2:\n" \
> > + _ASM_EXTABLE_UACCESS_ERR(1b, 2b, %w0) \
> > + : "+r" (ret), "+Q" (*uaddr), "=r" (oldval) \
> > + : "r" (oparg) \
> > + : "memory"); \
> > + \
> > + if (!ret) \
> > + *oval = oldval; \
> > + \
> > + return ret; \
> > +}
> > +
> > +LSUI_FUTEX_ATOMIC_OP(add, ldtadd, al)
> > +LSUI_FUTEX_ATOMIC_OP(or, ldtset, al)
> > +LSUI_FUTEX_ATOMIC_OP(andnot, ldtclr, al)
> > +LSUI_FUTEX_ATOMIC_OP(set, swpt, al)
> > +
> > +static __always_inline int
> > +__lsui_cmpxchg64(u64 __user *uaddr, u64 *oldval, u64 newval)
> > +{
> > + int ret = 0;
> > +
> > + asm volatile("// __lsui_cmpxchg64\n"
> > + __LSUI_PREAMBLE
> > +"1: casalt %x2, %x3, %1\n"
>
>
> How bizarre, they changed the order of the AL and T compared to SWPTAL.
> Fair enough...
>
> Also, I don't think you need the 'x' prefix on the 64-bit variables.

Right. I'll remove useless prefix.

>
> > +"2:\n"
> > + _ASM_EXTABLE_UACCESS_ERR(1b, 2b, %w0)
> > + : "+r" (ret), "+Q" (*uaddr), "+r" (*oldval)
> > + : "r" (newval)
> > + : "memory");
>
> Don't you need to update *oldval here if the CAS didn't fault?

No. if CAS doesn't make fault the oldval update already.

>
> > +
> > + return ret;
> > +}
> > +
> > +static __always_inline int
> > +__lsui_cmpxchg32(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
> > +{
> > + u64 __user *uaddr64;
> > + bool futex_on_lo;
> > + int ret = -EAGAIN, i;
> > + u32 other, orig_other;
> > + union {
> > + struct futex_on_lo {
> > + u32 val;
> > + u32 other;
> > + } lo_futex;
> > +
> > + struct futex_on_hi {
> > + u32 other;
> > + u32 val;
> > + } hi_futex;
> > +
> > + u64 raw;
> > + } oval64, orig64, nval64;
> > +
> > + uaddr64 = (u64 __user *) PTR_ALIGN_DOWN(uaddr, sizeof(u64));
> > + futex_on_lo = (IS_ALIGNED((unsigned long)uaddr, sizeof(u64)) ==
> > + IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN));
>
> Just make LSUI depend on !CPU_BIG_ENDIAN in Kconfig. The latter already
> depends on BROKEN and so we'll probably drop it soon anyway. There's
> certainly no need to care about it for new features and it should simplify
> the code you have here if you can assume little-endian.

Thanks. then I'll enable LSUI feature only !CPU_BIG_ENDIAN.

>
> > +
> > + for (i = 0; i < FUTEX_MAX_LOOPS; i++) {
> > + if (get_user(oval64.raw, uaddr64))
> > + return -EFAULT;
>
> Since oldval is passed to us as an argument, can we get away with a
> 32-bit get_user() here?

It's not a probelm. but is there any sigificant difference?

>
> > +
> > + nval64.raw = oval64.raw;
> > +
> > + if (futex_on_lo) {
> > + oval64.lo_futex.val = oldval;
> > + nval64.lo_futex.val = newval;
> > + } else {
> > + oval64.hi_futex.val = oldval;
> > + nval64.hi_futex.val = newval;
> > + }
> > +
> > + orig64.raw = oval64.raw;
> > +
> > + if (__lsui_cmpxchg64(uaddr64, &oval64.raw, nval64.raw))
> > + return -EFAULT;
> > +
> > + if (futex_on_lo) {
> > + oldval = oval64.lo_futex.val;
> > + other = oval64.lo_futex.other;
> > + orig_other = orig64.lo_futex.other;
> > + } else {
> > + oldval = oval64.hi_futex.val;
> > + other = oval64.hi_futex.other;
> > + orig_other = orig64.hi_futex.other;
> > + }
> > +
> > + if (other == orig_other) {
> > + ret = 0;
> > + break;
> > + }
> > + }
> > +
> > + if (!ret)
> > + *oval = oldval;
>
> Shouldn't we set *oval to the value we got back from the CAS?

Since it's a "success" case, the CAS return and oldval must be the same.
That's why it doesn't matter to use got back from the CAS.
Otherwise, it returns error and *oval doesn't matter for
futex_atomic_cmpxchg_inatomic().

>
> > +
> > + return ret;
> > +}
> > +
> > +static __always_inline int
> > +__lsui_futex_atomic_and(int oparg, u32 __user *uaddr, int *oval)
> > +{
> > + return __lsui_futex_atomic_andnot(~oparg, uaddr, oval);
>
> Please a comment about the bitwise negation of oparg here as we're undoing
> the one from the caller.

I see. Thanks!

>
> > +}
> > +
> > +static __always_inline int
> > +__lsui_futex_atomic_eor(int oparg, u32 __user *uaddr, int *oval)
> > +{
> > + u32 oldval, newval, val;
> > + int ret, i;
> > +
> > + /*
> > + * there are no ldteor/stteor instructions...
> > + */
> > + for (i = 0; i < FUTEX_MAX_LOOPS; i++) {
> > + if (get_user(oldval, uaddr))
> > + return -EFAULT;
> > +
> > + newval = oldval ^ oparg;
> > +
> > + ret = __lsui_cmpxchg32(uaddr, oldval, newval, &val);
> > + if (ret)
> > + return ret;
> > +
> > + if (val == oldval) {
> > + *oval = val;
> > + return 0;
> > + }
> > + }
> > +
> > + return -EAGAIN;
> > +}
> > +
> > +static __always_inline int
> > +__lsui_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
> > +{
> > + return __lsui_cmpxchg32(uaddr, oldval, newval, oval);
> > +}
> > +
> > +#define __lsui_llsc_body(op, ...) \
> > +({ \
> > + alternative_has_cap_likely(ARM64_HAS_LSUI) ? \
>
> This doesn't seem like it should be the "likely" case just yet?

Okay. I'll change with "unlikely"

--
Sincerely,
Yeoreum Yun