linux-kernel - Re: [PATCH v3 1/5] arm64: make SCTLR2

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Message-ID: <aKXlYhhrNp/8StK2@e133380.arm.com>
Date: Wed, 20 Aug 2025 16:10:26 +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,
	Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v3 1/5] arm64: make SCTLR2_EL1 accessible

Hi,

On Wed, Aug 13, 2025 at 01:01:14PM +0100, Yeoreum Yun wrote:
> When the kernel runs at EL1, and yet is booted at EL2,
> HCRX_EL2.SCTLR2En must be set to avoid trapping SCTLR2_EL1 accesses
> from EL1 to EL2.
> 
> Ensure this bit is set at the point of initialising EL2.
> 
> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> Reviewed-by: Marc Zyngier <maz@...nel.org>
> ---
>  arch/arm64/include/asm/el2_setup.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
> index 46033027510c..d755b4d46d77 100644
> --- a/arch/arm64/include/asm/el2_setup.h
> +++ b/arch/arm64/include/asm/el2_setup.h
> @@ -57,9 +57,15 @@
>          /* Enable GCS if supported */
>  	mrs_s	x1, SYS_ID_AA64PFR1_EL1
>  	ubfx	x1, x1, #ID_AA64PFR1_EL1_GCS_SHIFT, #4
> -	cbz	x1, .Lset_hcrx_\@
> +	cbz	x1, .Lskip_hcrx_GCSEn_\@
>  	orr	x0, x0, #HCRX_EL2_GCSEn
>  
> +.Lskip_hcrx_GCSEn_\@:
> +	mrs_s	x1, SYS_ID_AA64MMFR3_EL1
> +	ubfx	x1, x1, #ID_AA64MMFR3_EL1_SCTLRX_SHIFT, #4
> +	cbz	x1, .Lset_hcrx_\@
> +	orr	x0, x0, HCRX_EL2_SCTLR2En

Nit: prefix immediate operands with # please -- see usage elsewhere in
this file.

(This comes from the legacy AArch32 syntax and has never been required
by AArch64 assemblers, but it has become a tradition in the Linux arch
code...)

The only execptions to this rule are macros (mov_q, mrs_s etc. --
frequently they have an underscore in the name; "real" instructions
never do.)

> +
>  .Lset_hcrx_\@:

Maybe rename this label to .Lskip_hcrx_SCTLR2En_\@, so that people
don't have to keep renaming an existing label whenever they add
another block here.

>  	msr_s	SYS_HCRX_EL2, x0
>  .Lskip_hcrx_\@:

[...]

Cheers
---Dave