[PATCH v2] armv8: Handle EL2 Host mode

[Andre Przywara]

On Sat, 30 Jan 2021 22:31:06 +0100
Mark Kettenis <kettenis at openbsd.org> wrote:

Hi,

> On implementations that support VHE, the layout of the CPTR_EL2
> register depends on whether HCR_EL2.E2H is set.  Check this bit
> and and set the aprropriate bits to enable access to the FP/SIMD
> registers.  This allows U-Boot to run on systems that pass
> control to U-Boot in EL2 with EL2 Host mode enabled such as
> machine with Apple's M1 SoC.
> 
> Signed-off-by: Mark Kettenis <kettenis at openbsd.org>

Reviewed-by: Andre Przywara <andre.przywara at arm.com>

So architecturally this is correct, but I was wondering why you would
actually need this? I gave this a quick spin in QEMU (-cpu max),
hacking E2H to 1 very early in start.S. However loading and executing
the Linux kernel was fine even without this patch, so did you observe
an actual problem? Any kernel entered in EL2 would probably take care of
the E2H bit and CPTR itself (Linux sets E2H, FreeBSD clears it), so
is (your?) U-Boot actually using SIMD or VFP instructions itself?

Cheers,
Andre.

> ---
> 
> v2: rename label
> 
>  arch/arm/cpu/armv8/start.S | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/arm/cpu/armv8/start.S b/arch/arm/cpu/armv8/start.S
> index 662449156b..979dcfef35 100644
> --- a/arch/arm/cpu/armv8/start.S
> +++ b/arch/arm/cpu/armv8/start.S
> @@ -133,9 +133,15 @@ pie_fixup_done:
>  #endif
>  	b	0f
>  2:	set_vbar	vbar_el2, x0
> +	mrs	x0, hcr_el2
> +	tbnz	x0, #34, el2_vhe		/* HCR_EL2.E2H */
>  	mov	x0, #0x33ff
>  	msr	cptr_el2, x0			/* Enable FP/SIMD */
>  	b	0f
> +el2_vhe:
> +	mov	x0, #3 << 20
> +	msr	cptr_el2, x0			/* Enable FP/SIMD */
> +	b	0f
>  1:	set_vbar	vbar_el1, x0
>  	mov	x0, #3 << 20
>  	msr	cpacr_el1, x0			/* Enable FP/SIMD */