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

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Message-ID: <aKYHK1tYEji3jKWJ@e129823.arm.com>
Date: Wed, 20 Aug 2025 18:34:35 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Dave Martin <Dave.Martin@....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 Dave,

> 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.

Thanks for your suggestion :)

>
> > 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.

I see. Thanks!

>
> >  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.

You're right. there's no usage anyware except here.
I'll change this.

Thanks!

>
>
> > +{
> > +	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

--
Sincerely,
Yeoreum Yun