linux-kernel - Re: [PATCH v3 3/5] arm64: save/restore SCTLR2_EL1 when cpu

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Message-ID: <aKYEcwKODLsevnTj@e129823.arm.com>
Date: Wed, 20 Aug 2025 18:22:59 +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 3/5] arm64: save/restore SCTLR2_EL1 when
 cpu_suspend()/resume()

Hi Dave,

> On Wed, Aug 13, 2025 at 01:01:16PM +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 (i.e) FEAT_PAuth_LR or FEAT_CPA/CPA2
> >
> > Save and restore SCTLR2_EL1 when cpu_suspend() and resume().
> > so that configured value can sustain consistency before suspend and
> > after resume.
> >
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> > ---
> >  arch/arm64/include/asm/suspend.h |  2 +-
> >  arch/arm64/mm/proc.S             | 26 ++++++++++++++++++--------
> >  2 files changed, 19 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/suspend.h b/arch/arm64/include/asm/suspend.h
> > index 0cde2f473971..eb60c9735553 100644
> > --- a/arch/arm64/include/asm/suspend.h
> > +++ b/arch/arm64/include/asm/suspend.h
> > @@ -2,7 +2,7 @@
> >  #ifndef __ASM_SUSPEND_H
> >  #define __ASM_SUSPEND_H
> >
> > -#define NR_CTX_REGS 13
> > +#define NR_CTX_REGS 14
> >  #define NR_CALLEE_SAVED_REGS 12
> >
> >  /*
> > diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> > index 8c75965afc9e..f297bea7103b 100644
> > --- a/arch/arm64/mm/proc.S
> > +++ b/arch/arm64/mm/proc.S
> > @@ -87,8 +87,14 @@ SYM_FUNC_START(cpu_do_suspend)
> >  	mrs	x9, mdscr_el1
> >  	mrs	x10, oslsr_el1
> >  	mrs	x11, sctlr_el1
> > -	get_this_cpu_offset x12
> > -	mrs	x13, sp_el0
> > +alternative_if_not ARM64_HAS_SCTLR2
> > +	mov	x12, xzr
>
> Looking at this, maybe it can just be a nop for the !ARM64_HAS_SCTLR2
> case.
>
> (So, alternative_if ... alternative_if_else_nop_endif, similarly to
> what you have in cpu_do_resume.)
>
> The memory used to save this state should not be accessible to anything
> less privileged than the kernel anyway, so leaking whatever was in x12
> does not really feel like a concern...

Right. I'll change this.

Thanks!

>
>
> > +alternative_else
> > +	mrs_s	x12, SYS_SCTLR2_EL1
> > +alternative_endif
> > +	get_this_cpu_offset x13
> > +	mrs	x14, sp_el0
> > +
> >  	stp	x2, x3, [x0]
> >  	stp	x4, x5, [x0, #16]
> >  	stp	x6, x7, [x0, #32]
> > @@ -99,7 +105,7 @@ SYM_FUNC_START(cpu_do_suspend)
> >  	 * Save x18 as it may be used as a platform register, e.g. by shadow
> >  	 * call stack.
> >  	 */
> > -	str	x18, [x0, #96]
> > +	stp	x14, x18, [x0, #96]
> >  	ret
> >  SYM_FUNC_END(cpu_do_suspend)
> >
> > @@ -120,8 +126,8 @@ SYM_FUNC_START(cpu_do_resume)
> >  	 * the buffer to minimize the risk of exposure when used for shadow
> >  	 * call stack.
> >  	 */
> > -	ldr	x18, [x0, #96]
> > -	str	xzr, [x0, #96]
> > +	ldp	x15, x18, [x0, #96]
> > +	str	xzr, [x0, #104]
> >  	msr	tpidr_el0, x2
> >  	msr	tpidrro_el0, x3
> >  	msr	contextidr_el1, x4
> > @@ -136,8 +142,12 @@ SYM_FUNC_START(cpu_do_resume)
> >  	msr	mdscr_el1, x10
> >
> >  	msr	sctlr_el1, x12
> > -	set_this_cpu_offset x13
> > -	msr	sp_el0, x14
> > +alternative_if ARM64_HAS_SCTLR2
> > +	msr_s	SYS_SCTLR2_EL1, x13
> > +alternative_else_nop_endif
> > +
> > +	set_this_cpu_offset x14
> > +	msr	sp_el0, x15
> >  	/*
> >  	 * Restore oslsr_el1 by writing oslar_el1
> >  	 */
> > @@ -151,7 +161,7 @@ alternative_if ARM64_HAS_RAS_EXTN
> >  	msr_s	SYS_DISR_EL0, xzr
> >  alternative_else_nop_endif
> >
> > -	ptrauth_keys_install_kernel_nosync x14, x1, x2, x3
> > +	ptrauth_keys_install_kernel_nosync x15, x1, x2, x3
> >  	isb
> >  	ret
> >  SYM_FUNC_END(cpu_do_resume)
> > --
> > LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}
>
> Otherwise, this looks OK to me.
>
> Cheers
> ---Dave

--
Sincerely,
Yeoreum Yun