linux-kernel - Re: [PATCH v2 1/6] arm64: make SCTLR2

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Message-ID: <86sehx92ev.wl-maz@kernel.org>
Date: Mon, 11 Aug 2025 18:46:48 +0100
From: Marc Zyngier <maz@...nel.org>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: catalin.marinas@....com,
	will@...nel.org,
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Subject: Re: [PATCH v2 1/6] arm64: make SCTLR2_EL1 accessible

On Mon, 11 Aug 2025 17:33:35 +0100,
Yeoreum Yun <yeoreum.yun@....com> wrote:
> 
> make SCTLR2_EL1 accssible to initilise it.

nit: "accessible", "initialise".

This could deserve a slightly less terse message, so that someone who
is not very much versed into the boring details of the architecture
can make sense of this patch. Because, frankly, if you can access
HCRX_EL2, why can't you access SCTLR2_EL1? You know why, I know why,
but hardly anyone else does.

I'd suggest something along the lines of:

"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."

which at least explains why we're doing this.

> 
> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> ---
>  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
> +
>  .Lset_hcrx_\@:
>  	msr_s	SYS_HCRX_EL2, x0
>  .Lskip_hcrx_\@:

With that fixed,

Reviewed-by: Marc Zyngier <maz@...nel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.