linux-kernel - Re: [PATCH v3 5/5] arm64: make the per-task SCTLR2

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Message-ID: <aKXlqsnLyTL6lsh5@e133380.arm.com>
Date: Wed, 20 Aug 2025 16:11:38 +0100
From: Dave Martin <Dave.Martin@....com>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: catalin.marinas@....com, will@...nel.org, broonie@...nel.org,
	oliver.upton@...ux.dev, anshuman.khandual@....com, robh@...nel.org,
	james.morse@....com, mark.rutland@....com, joey.gouly@....com,
	ahmed.genidi@....com, kevin.brodsky@....com,
	scott@...amperecomputing.com, mbenes@...e.cz,
	james.clark@...aro.org, frederic@...nel.org, rafael@...nel.org,
	pavel@...nel.org, ryan.roberts@....com, suzuki.poulose@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 5/5] arm64: make the per-task SCTLR2_EL1

Hi,

On Wed, Aug 13, 2025 at 01:01:18PM +0100, Yeoreum Yun wrote:
> SCTLR2_EL1 register is optional starting from ARMv8.8/ARMv9.3,
> and becomes mandatory from ARMv8.9/ARMv9.4
> and serveral architectural feature are controled by bits in
> these registers and some of bits could be configurable per task
> not globally -- i.e) FEAT_CPA2 related field and etc.
> 
> For future usage of these fields, make the per-task SCTLR2_EL1.

It is worth pointing out the impact of this: for platforms without
FEAT_SCTLR2 support, there is no functional change and minimal
performance overhead.

> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> ---
>  arch/arm64/include/asm/processor.h | 5 +++++
>  arch/arm64/kernel/process.c        | 9 +++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index 61d62bfd5a7b..2c962816de70 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -184,6 +184,7 @@ struct thread_struct {
>  	u64			mte_ctrl;
>  #endif
>  	u64			sctlr_user;
> +	u64			sctlr2_user;
>  	u64			svcr;
>  	u64			tpidr2_el0;
>  	u64			por_el0;
> @@ -258,6 +259,9 @@ static inline void task_set_sve_vl_onexec(struct task_struct *task,
>  	(SCTLR_ELx_ENIA | SCTLR_ELx_ENIB | SCTLR_ELx_ENDA | SCTLR_ELx_ENDB |   \
>  	 SCTLR_EL1_TCF0_MASK)
>  
> +#define SCTLR2_USER_MASK	\
> +	(SCTLR2_EL1_EnPACM0 | SCTLR2_EL1_CPTA0 | SCTLR2_EL1_CPTM0)
> +

The kernel doesn't know about any of these features, yet.

It's probably better to make this 0 for this patch series, and add bits
to this mask only when they are needed / used.

>  static inline void arch_thread_struct_whitelist(unsigned long *offset,
>  						unsigned long *size)
>  {
> @@ -370,6 +374,7 @@ struct task_struct;
>  unsigned long __get_wchan(struct task_struct *p);
>  
>  void update_sctlr_el1(u64 sctlr);
> +void update_sctlr2_el1(u64 sctlr2);

Is this function used outside process.c yet?  If not, you can drop this
declaration and [... below ...]
>  
>  /* Thread switching */
>  extern struct task_struct *cpu_switch_to(struct task_struct *prev,
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 96482a1412c6..9191180c4875 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -698,6 +698,11 @@ void update_sctlr_el1(u64 sctlr)
>  	isb();
>  }
>  
> +void update_sctlr2_el1(u64 sctlr2)

[...] make the function static here.


> +{
> +	sysreg_clear_set_s(SYS_SCTLR2_EL1, SCTLR2_USER_MASK, sctlr2);
> +}
> +
>  /*
>   * Thread switching.
>   */
> @@ -737,6 +742,10 @@ struct task_struct *__switch_to(struct task_struct *prev,
>  	if (prev->thread.sctlr_user != next->thread.sctlr_user)
>  		update_sctlr_el1(next->thread.sctlr_user);
>  
> +	if (alternative_has_cap_unlikely(ARM64_HAS_SCTLR2) &&
> +	    prev->thread.sctlr2_user != next->thread.sctlr2_user)
> +		update_sctlr2_el1(next->thread.sctlr2_user);
> +
>  	/* the actual thread switch */
>  	last = cpu_switch_to(prev, next);

[...]

Otherwise, I guess this looks OK.

Cheers
---Dave