[PATCH 2/2] arm64: Cope with CPUs stuck in VHE mode

[Will Deacon]

On Thu, Mar 25, 2021 at 12:47:21PM +0000, Marc Zyngier wrote:
> It seems that the CPUs part of the SoC known as Apple M1 have the
> terrible habit of being stuck with HCR_EL2.E2H==1, in violation
> of the architecture.
> 
> Try and work around this deplorable state of affairs by detecting
> the stuck bit early and short-circuit the nVHE dance. Additional
> filtering code ensures that attempts at switching to nVHE from
> the command-line are also ignored.
> 
> It is still unknown whether there are many more such nuggets
> to be found...
> 
> Reported-by: Hector Martin <marcan at marcan.st>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/kernel/head.S           | 33 +++++++++++++++++++++++++++---
>  arch/arm64/kernel/hyp-stub.S       | 15 ++++++++++----
>  arch/arm64/kernel/idreg-override.c | 13 +++++++++++-
>  3 files changed, 53 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 840bda1869e9..db2de5b8f3d9 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -477,14 +477,13 @@ EXPORT_SYMBOL(kimage_vaddr)
>   * booted in EL1 or EL2 respectively.
>   */
>  SYM_FUNC_START(init_kernel_el)
> -	mov_q	x0, INIT_SCTLR_EL1_MMU_OFF
> -	msr	sctlr_el1, x0
> -
>  	mrs	x0, CurrentEL
>  	cmp	x0, #CurrentEL_EL2
>  	b.eq	init_el2
>  
>  SYM_INNER_LABEL(init_el1, SYM_L_LOCAL)
> +	mov_q	x0, INIT_SCTLR_EL1_MMU_OFF
> +	msr	sctlr_el1, x0
>  	isb
>  	mov_q	x0, INIT_PSTATE_EL1
>  	msr	spsr_el1, x0
> @@ -504,6 +503,34 @@ SYM_INNER_LABEL(init_el2, SYM_L_LOCAL)
>  	msr	vbar_el2, x0
>  	isb
>  
> +	/*
> +	 * 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!
> +	 */
> +	mrs	x0, hcr_el2
> +	and	x0, x0, #HCR_E2H
> +	cbz	x0, 1f
> +
> +	/* Switching to VHE requires a sane SCTLR_EL1 as a start */
> +	mov_q	x0, INIT_SCTLR_EL1_MMU_OFF
> +	msr_s	SYS_SCTLR_EL12, x0
> +
> +	/*
> +	 * Force an eret into a helper "function", and let it return
> +	 * to our original caller... This makes sure that we have
> +	 * initialised the basic PSTATE state.
> +	 */
> +	mov	x0, #INIT_PSTATE_EL2
> +	msr	spsr_el1, x0
> +	adr_l	x0, stick_to_vhe
> +	msr	elr_el1, x0
> +	eret

What does this do if CONFIG_VHE=n on one of these CPUs?

Will