[v17] support FEAT_LSUI

[PATCH v17 0/8] support FEAT_LSUI

Posted by Yeoreum Yun 3 weeks ago

Since Armv9.6, FEAT_LSUI supplies the load/store instructions for
previleged level to access to access user memory without clearing
PSTATE.PAN bit.

This patchset support FEAT_LSUI and applies it mainly in
futex atomic operation and others.

This patch based on v7.0-rc3


Patch History
==============
from v16 to v17
  - remove loops in __lsui_cmpxchg32().
  - modify some comments and move them to commit message.
  - https://lore.kernel.org/all/20260312175243.1593864-1-yeoreum.yun@arm.com/

from v15 to v16:
  - remove positional operand in futex asm.
  - rebased to v7.0-rc3
  - https://lore.kernel.org/all/20260227151705.1275328-1-yeoreum.yun@arm.com/

from v14 to v15:
  - replace caslt to cast
  - cleanup the patch
  - https://lore.kernel.org/all/20260225182708.3225211-1-yeoreum.yun@arm.com/

from v13 to v14:
  - add LSUI config check in cpucap_is_possible()
  - fix build failure with clang-19
  - https://lore.kernel.org/all/20260223174802.458411-1-yeoreum.yun@arm.com/

from v12 to v13:
  - rebase to v7.0-rc1
  - apply CASLT for swapping guest descriptor
  - remove has_lsui() for checking cpu feature.
  - simplify __lsui_cmpxchg32() according to @Catalin's suggestion.
  - use uaccess_ttbr0_enable()/disable() for LSUI instructions.
  - https://lore.kernel.org/all/aYWuqTqM5MvudI5V@e129823.arm.com/

from v11 to v12:
  - rebase to v6.19-rc6
  - add CONFIG_ARM64_LSUI
  - enable LSUI when !CPU_BIG_ENDIAN and PAN presents.
  - drop the swp emulation with LSUI insns instead, disable it
    when LSUI presents.
  - some of small fixes (useless prefix and suffix and etc).
  - https://lore.kernel.org/all/20251214112248.901769-1-yeoreum.yun@arm.com/

from v10 to v11:
  - rebase to v6.19-rc1
  - use cast instruction to emulate deprecated swpb instruction
  - https://lore.kernel.org/all/20251103163224.818353-1-yeoreum.yun@arm.com/

from v9 to v10:
  - apply FEAT_LSUI to user_swpX emulation.
  - add test coverage for LSUI bit in ID_AA64ISAR3_EL1
  - rebase to v6.18-rc4
  - https://lore.kernel.org/all/20250922102244.2068414-1-yeoreum.yun@arm.com/

from v8 to v9:
  - refotoring __lsui_cmpxchg64()
  - rebase to v6.17-rc7
  - https://lore.kernel.org/all/20250917110838.917281-1-yeoreum.yun@arm.com/

from v7 to v8:
  - implements futex_atomic_eor() and futex_atomic_cmpxchg() with casalt
    with C helper.
  - Drop the small optimisation on ll/sc futex_atomic_set operation.
  - modify some commit message.
  - https://lore.kernel.org/all/20250816151929.197589-1-yeoreum.yun@arm.com/

from v6 to v7:
  - wrap FEAT_LSUI with CONFIG_AS_HAS_LSUI in cpufeature
  - remove unnecessary addition of indentation.
  - remove unnecessary mte_tco_enable()/disable() on LSUI operation.
  - https://lore.kernel.org/all/20250811163635.1562145-1-yeoreum.yun@arm.com/

from v5 to v6:
  - rebase to v6.17-rc1
  - https://lore.kernel.org/all/20250722121956.1509403-1-yeoreum.yun@arm.com/

from v4 to v5:
  - remove futex_ll_sc.h futext_lsui and lsui.h and move them to futex.h
  - reorganize the patches.
  - https://lore.kernel.org/all/20250721083618.2743569-1-yeoreum.yun@arm.com/

from v3 to v4:
  - rebase to v6.16-rc7
  - modify some patch's title.
  - https://lore.kernel.org/all/20250617183635.1266015-1-yeoreum.yun@arm.com/

from v2 to v3:
  - expose FEAT_LSUI to guest
  - add help section for LSUI Kconfig
  - https://lore.kernel.org/all/20250611151154.46362-1-yeoreum.yun@arm.com/

from v1 to v2:
  - remove empty v9.6 menu entry
  - locate HAS_LSUI in cpucaps in order
  - https://lore.kernel.org/all/20250611104916.10636-1-yeoreum.yun@arm.com/


Yeoreum Yun (8):
  arm64: cpufeature: add FEAT_LSUI
  KVM: arm64: expose FEAT_LSUI to guest
  KVM: arm64: kselftest: set_id_regs: add test for FEAT_LSUI
  arm64: futex: refactor futex atomic operation
  arm64: futex: support futex with FEAT_LSUI
  arm64: armv8_deprecated: disable swp emulation when FEAT_LSUI present
  KVM: arm64: use CAST instruction for swapping guest descriptor
  arm64: Kconfig: add support for LSUI

 arch/arm64/Kconfig                            |  20 ++
 arch/arm64/include/asm/cpucaps.h              |   2 +
 arch/arm64/include/asm/futex.h                | 307 ++++++++++++++----
 arch/arm64/include/asm/lsui.h                 |  27 ++
 arch/arm64/kernel/armv8_deprecated.c          |  16 +
 arch/arm64/kernel/cpufeature.c                |  10 +
 arch/arm64/kvm/at.c                           |  34 +-
 arch/arm64/kvm/sys_regs.c                     |   3 +-
 arch/arm64/tools/cpucaps                      |   1 +
 .../testing/selftests/kvm/arm64/set_id_regs.c |   1 +
 10 files changed, 361 insertions(+), 60 deletions(-)
 create mode 100644 arch/arm64/include/asm/lsui.h

--
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}

Re: (subset) [PATCH v17 0/8] support FEAT_LSUI

Posted by Catalin Marinas 1 week, 1 day ago

On Sat, 14 Mar 2026 17:51:25 +0000, Yeoreum Yun wrote:
> Since Armv9.6, FEAT_LSUI supplies the load/store instructions for
> previleged level to access to access user memory without clearing
> PSTATE.PAN bit.
> 
> This patchset support FEAT_LSUI and applies it mainly in
> futex atomic operation and others.
> 
> [...]

Applied to arm64 (for-next/feat_lsui), thanks!

[6/8] arm64: armv8_deprecated: disable swp emulation when FEAT_LSUI present
      (commit e223258ed8a6)

-- 
Catalin

Re: (subset) [PATCH v17 0/8] support FEAT_LSUI

Posted by Catalin Marinas 1 week, 2 days ago

On Sat, 14 Mar 2026 17:51:25 +0000, Yeoreum Yun wrote:
> Since Armv9.6, FEAT_LSUI supplies the load/store instructions for
> previleged level to access to access user memory without clearing
> PSTATE.PAN bit.
> 
> This patchset support FEAT_LSUI and applies it mainly in
> futex atomic operation and others.
> 
> [...]

Applied to arm64 (for-next/feat_lsui), thanks!

I decided to drop patch [6/8] (arm64: armv8_deprecated: disable swp
emulation when FEAT_LSUI present). The way FEAT_LSUI support looks now,
we still have uaccess_enable_privileged() working properly and we could
even support SWP emulation using exclusives. While it's highly unlikely
to see both 32-bit EL0 and FEAT_LSUI in practice, models may support the
combination and disabling SWP emulation feels pretty artificial.

[1/8] arm64: cpufeature: add FEAT_LSUI
      https://git.kernel.org/arm64/c/7181f718cb0f
[2/8] KVM: arm64: expose FEAT_LSUI to guest
      https://git.kernel.org/arm64/c/f6bff18d05ed
[3/8] KVM: arm64: kselftest: set_id_regs: add test for FEAT_LSUI
      https://git.kernel.org/arm64/c/42550d7d8aa6
[4/8] arm64: futex: refactor futex atomic operation
      https://git.kernel.org/arm64/c/eaa3babcceaa
[5/8] arm64: futex: support futex with FEAT_LSUI
      https://git.kernel.org/arm64/c/44adf2bf40ef
[7/8] KVM: arm64: use CAST instruction for swapping guest descriptor
      https://git.kernel.org/arm64/c/16dbe77a5be2
[8/8] arm64: Kconfig: add support for LSUI
      https://git.kernel.org/arm64/c/377609ae8b6a

-- 
Catalin

Re: (subset) [PATCH v17 0/8] support FEAT_LSUI

Posted by Yeoreum Yun 1 week, 2 days ago

Hi Catalin,

> On Sat, 14 Mar 2026 17:51:25 +0000, Yeoreum Yun wrote:
> > Since Armv9.6, FEAT_LSUI supplies the load/store instructions for
> > previleged level to access to access user memory without clearing
> > PSTATE.PAN bit.
> >
> > This patchset support FEAT_LSUI and applies it mainly in
> > futex atomic operation and others.
> >
> > [...]
>
> Applied to arm64 (for-next/feat_lsui), thanks!

Thanks!

>
> I decided to drop patch [6/8] (arm64: armv8_deprecated: disable swp
> emulation when FEAT_LSUI present). The way FEAT_LSUI support looks now,
> we still have uaccess_enable_privileged() working properly and we could
> even support SWP emulation using exclusives. While it's highly unlikely
> to see both 32-bit EL0 and FEAT_LSUI in practice,

This is one of decisive reason to drop the swp emulation with LSUI
(https://lore.kernel.org/all/aXDbBKhE1SdCW6q4@willie-the-truck/)

However,

> models may support the
> combination and disabling SWP emulation feels pretty artificial.

But I'm not sure this is a sufficient rationale for supporting SWP with LSUI,
since it's highly unlikely to encounter a real CPU that supports both 32-bit EL0
and FEAT_LSUI.

Anyway, it's fair enough to drop 6/8 right now.
But I appreciate whether it would be good to support SWP emulation with
LSUI so that let me respin for it with the former patch.

[...]

--
Sincerely,
Yeoreum Yun

Re: (subset) [PATCH v17 0/8] support FEAT_LSUI

Posted by Catalin Marinas 1 week, 1 day ago

On Fri, Mar 27, 2026 at 01:56:11PM +0000, Yeoreum Yun wrote:
> > I decided to drop patch [6/8] (arm64: armv8_deprecated: disable swp
> > emulation when FEAT_LSUI present). The way FEAT_LSUI support looks now,
> > we still have uaccess_enable_privileged() working properly and we could
> > even support SWP emulation using exclusives. While it's highly unlikely
> > to see both 32-bit EL0 and FEAT_LSUI in practice,
> 
> This is one of decisive reason to drop the swp emulation with LSUI
> (https://lore.kernel.org/all/aXDbBKhE1SdCW6q4@willie-the-truck/)

Ah, I forgot about this discussion. It's a valid point, I just thought
it's unnecessary given that __uaccess_disable_hw_pan() still works.

If we want strict no PAN, I can add it back (really small patch). I
wonder whether we should also add a
WARN_ON_ONCE(cpus_have_final_cap(ARM64_HAS_LSUI)) to the pan disabling
function. Not urgent though.

-- 
Catalin