linux-kernel - Re: [PATCH v3 4/5] arm64: initialise SCTLR2_EL1 at cpu_soft

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Message-ID: <aKXlnZ8HXHBZMj4T@e133380.arm.com>
Date: Wed, 20 Aug 2025 16:11:25 +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 4/5] arm64: initialise SCTLR2_EL1 at cpu_soft_restart()

Hi,

On Wed, Aug 13, 2025 at 01:01:17PM +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.
> 
> Before, launching new kernel via kexec, initialise SCTLR2_EL1 explicitly.
> 
> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> ---
>  arch/arm64/kernel/cpu-reset.S | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/arm64/kernel/cpu-reset.S b/arch/arm64/kernel/cpu-reset.S
> index c87445dde674..123564af345b 100644
> --- a/arch/arm64/kernel/cpu-reset.S
> +++ b/arch/arm64/kernel/cpu-reset.S
> @@ -37,6 +37,12 @@ SYM_TYPED_FUNC_START(cpu_soft_restart)
>  	 * regime if HCR_EL2.E2H == 1
>  	 */
>  	msr	sctlr_el1, x12
> +
> +alternative_if ARM64_HAS_SCTLR2
> +	mov_q	x12, INIT_SCTLR2_EL1
> +	msr_s	SYS_SCTLR2_EL1, x12
> +alternative_else_nop_endif
> +

It would be better to do this based on the ID regs.

Although the previous kernel _shouldn't_ have used SCTLR2 if the
capability ARM64_HAS_SCTLR2 did not get enabled, it would be better to
enforce a clean state here for the new kernel.

If so, maybe one of the macros that you already defined can be used
here?  (But it's also fine to open-code it.)

>  	isb
>  
>  	cbz	x0, 1f				// el2_switch?

[...]

In the case where the el2_switch argument in non-zero, don't we also
need to do something to reinitialise SCTLR2_EL2 after switching back
to EL2, in the HVC_SOFT_RESTART handler?

Maybe I missed something.

Cheers
---Dave