Re: [PATCH 0/7] arm64: Batch PSTATE.TCO handling in kernel nofault loops

[Muhammad Usama Anjum <usama.anjum@arm.com>]

Reminder

On 24/08/2026 5:04 pm, Muhammad Usama Anjum wrote:
> With Hardware Tag-Based KASAN in asynchronous or asymmetric mode, kernel
> nofault loops currently set and clear PSTATE.TCO around every access.
> 
> Introduce bare nofault accessors, batching hooks, and an internal scope
> guard. Convert maccess page-fault cleanup to scoped form. Skip
> page-fault setup for zero-sized kernel nofault copies and zero-length
> BPF string operations. Then use bare primitives in the maccess and BPF
> loops. For non-empty operations, this reduces the code-derived dynamic
> MSR TCO execution count from 2N to 2. Two-string BPF comparisons fall
> from 4N to 2.
> 
> The BPF changes and an earlier maccess implementation with explicit
> cleanup were tested with QEMU arm64 using Hardware Tag-Based KASAN in
> synchronous, asynchronous, and asymmetric modes. All nine arm64 MTE
> kselftests passed in each mode, as did the 138 focused BPF string_kfuncs
> and varlen subtests. The focused BPF tests also passed with the default
> non-MTE arm64 CPU model. Both final maccess scoped-guard patches were
> arm64 cross-built. Runtime tests have not been rerun after that
> conversion.
> 
> Muhammad Usama Anjum (7):
>   arm64: uaccess: Add batched kernel nofault accessors
>   uaccess: Add scope guard for bare kernel nofault regions
>   maccess: Skip setup for zero-sized kernel nofault copies
>   maccess: Use a scoped guard for page faults
>   maccess: Batch TCO handling in kernel nofault loops
>   bpf: Skip setup for zero-length string kfunc operations
>   bpf: Batch TCO handling in string kfuncs
> 
>  arch/arm64/include/asm/uaccess.h | 71 ++++++++++++++++++++++++--------
>  include/linux/uaccess.h          | 20 +++++++++
>  kernel/bpf/helpers.c             | 41 ++++++++++++------
>  mm/maccess.c                     | 68 ++++++++++++++++--------------
>  4 files changed, 139 insertions(+), 61 deletions(-)
> 


-- 
Thanks,
Usama