Re: [PATCH 2/5] arm64: Treat HCR_EL2.E2H as RES1 when ID_AA64MMFR4_EL1.E2H0 is negative

[Oliver Upton <oupton@kernel.org>]

On Wed, Jul 01, 2026 at 08:43:39PM +0000, Colton Lewis wrote:
> From: Marc Zyngier <maz@kernel.org>
> 
> [ Upstream commit 3944382fa6f22b54bc3624c9657b98ec34b5ba59 ]

What is this? I have the commit in question as

3944382fa6f22b54fd399632b1af92c28123979b

> For CPUs that have ID_AA64MMFR4_EL1.E2H0 as negative, it is important
> to avoid the boot path that sets HCR_EL2.E2H=0. Fortunately, we
> already have this path to cope with fruity CPUs.
> 
> Tweak init_el2 to look at ID_AA64MMFR4_EL1.E2H0 first.
> 
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Link: https://lore.kernel.org/r/20240122181344.258974-8-maz@kernel.org
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/kernel/head.S | 23 +++++++++++++++--------
>  1 file changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 6517bf2644a08..e32c8dd0b17a7 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -589,25 +589,32 @@ SYM_INNER_LABEL(init_el2, SYM_L_LOCAL)
>  	mov_q	x1, INIT_SCTLR_EL1_MMU_OFF
>  
>  	/*
> -	 * Fruity CPUs seem to have HCR_EL2.E2H set to RES1,
> -	 * making it impossible to start in nVHE mode. Is that
> -	 * compliant with the architecture? Absolutely not!
> +	 * Compliant CPUs advertise their VHE-onlyness with
> +	 * ID_AA64MMFR4_EL1.E2H0 < 0. HCR_EL2.E2H can be
> +	 * RES1 in that case.
> +	 *
> +	 * Fruity CPUs seem to have HCR_EL2.E2H set to RES1, but
> +	 * don't advertise it (they predate this relaxation).
>  	 */
> +	mrs_s	x0, SYS_ID_AA64MMFR4_EL1
> +	ubfx	x0, x0, #ID_AA64MMFR4_EL1_E2H0_SHIFT, #ID_AA64MMFR4_EL1_E2H0_WIDTH
> +	tbnz	x0, #(ID_AA64MMFR4_EL1_E2H0_SHIFT + ID_AA64MMFR4_EL1_E2H0_WIDTH - 1), 1f
> +
>  	mrs	x0, hcr_el2
>  	and	x0, x0, #HCR_E2H
> -	cbz	x0, 1f
> -
> +	cbz	x0, 2f
> +1:
>  	/* Set a sane SCTLR_EL1, the VHE way */
>  	pre_disable_mmu_workaround
>  	msr_s	SYS_SCTLR_EL12, x1
>  	mov	x2, #BOOT_CPU_FLAG_E2H
> -	b	2f
> +	b	3f
>  
> -1:
> +2:
>  	pre_disable_mmu_workaround
>  	msr	sctlr_el1, x1
>  	mov	x2, xzr
> -2:
> +3:
>  	__init_el2_nvhe_prepare_eret
>  
>  	mov	w0, #BOOT_CPU_MODE_EL2
> -- 
> 2.55.0.rc2.803.g1fd1e6609c-goog
> 
>