Subject: Re: [PATCH V2] arm64: Disable EL2 traps for BRBE instructions executed in EL1

From: Marc Zyngier <[email-masked]>
Date: Mon, 15 May 2023 14:42:08 +0100

On Mon, 15 May 2023 11:53:28 +0100,
Anshuman Khandual <[email-masked]> wrote:
> 
> This disables EL2 traps for BRBE instructions executed in EL1. This would
> enable BRBE to be configured and used successfully in the guest kernel.
> While here, this updates Documentation/arm64/booting.rst as well.
> 
> Cc: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: Mark Brown <[email-masked]>
> Cc: Marc Zyngier <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Signed-off-by: Anshuman Khandual <[email-masked]>
> ---
> This patch applies on v6.4-rc2
> 
> Changes in V2:
> 
> - Updated Documentation/arm64/booting.rst
> 
> Changes in V1:
> 
> https://lore.kernel.org/all/[email-masked]/
> 
>  Documentation/arm64/booting.rst    |  8 ++++++++
>  arch/arm64/include/asm/el2_setup.h | 10 ++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/Documentation/arm64/booting.rst b/Documentation/arm64/booting.rst
> index ffeccdd6bdac..cb9e151f6928 100644
> --- a/Documentation/arm64/booting.rst
> +++ b/Documentation/arm64/booting.rst
> @@ -379,6 +379,14 @@ Before jumping into the kernel, the following conditions must be met:
>  
>      - SMCR_EL2.EZT0 (bit 30) must be initialised to 0b1.
>  
> +  For CPUs with the Branch Record Buffer Extension (FEAT_BRBE):
> +
> + - If the kernel is entered at EL1 and EL2 is present:
> +
> +    - HFGITR_EL2.nBRBINJ (bit 55) must be initialised to 0b1.
> +
> +    - HFGITR_EL2.nBRBIALL (bit 56) must be initialised to 0b1.
> +
>  The requirements described above for CPU mode, caches, MMUs, architected
>  timers, coherency and system registers apply to all CPUs.  All CPUs must
>  enter the kernel in the same exception level.  Where the values documented
> diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
> index 037724b19c5c..06bf321a17be 100644
> --- a/arch/arm64/include/asm/el2_setup.h
> +++ b/arch/arm64/include/asm/el2_setup.h
> @@ -161,6 +161,16 @@
>  	msr_s	SYS_HFGWTR_EL2, x0
>  	msr_s	SYS_HFGITR_EL2, xzr
>  
> +	mrs	x1, id_aa64dfr0_el1
> +	ubfx	x1, x1, #ID_AA64DFR0_EL1_BRBE_SHIFT, #4
> +	cbz	x1, .Lskip_brbe_\@
> +
> +	mov	x0, xzr
> +	orr	x0, x0, #HFGITR_EL2_nBRBIALL
> +	orr	x0, x0, #HFGITR_EL2_nBRBINJ
> +	msr_s	SYS_HFGITR_EL2, x0

This will break badly if someone inserts something between this hunk
and the initial setting of HFGITR_EL2. I'd really prefer a RMW
approach.  It's not that this code has to be optimised anyway.

	M.

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

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