Re: [PATCH] efi/arm: enable CP15 DMB instructions before cleaning the cache

[Marc Zyngier <marc.zyngier@arm.com>]

Hi Ard,

On Fri, 29 Mar 2019 18:24:18 +0000,
Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> 
> The EFI stub is entered with the caches and MMU enabled by the
> firmware, and once the stub is ready to hand over to the decompressor,
> we clean and disable the caches.
> 
> The cache clean routines use CP15 barrier instructions, which can be
> disabled via SCTLR. Normally, when using the provided cache handling
> routines to enable the caches and MMU, this bit is enabled as well.
> However, but since we entered the stub with the caches already enabled,
> this routine is not executed before we call the cache clean routines,
> resulting in undefined instruction exceptions if the firmware never
> enabled this bit.
> 
> So set the bit explicitly in the EFI entry code.
> 
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  arch/arm/boot/compressed/head.S | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> index 6c7ccb428c07..62a49356fca3 100644
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -1438,6 +1438,16 @@ ENTRY(efi_stub_entry)
>  
>  		@ Preserve return value of efi_entry() in r4
>  		mov	r4, r0
> +
> +		@ our cache maintenance code relies on CP15 barrier instructions
> +		@ but since we arrived here with the MMU and caches configured
> +		@ by UEFI, we must ensure that the use of those instructions is
> +		@ enabled in the SCTLR register, since we never executed our own
> +		@ cache enable routine, which is normally in charge of this.
> +		mrc	p15, 0, r1, c1, c0, 0	@ read SCTLR
> +		orr	r1, r1, #(1 << 5)	@ CP15 barrier instructions
> +		mcr	p15, 0, r1, c1, c0, 0	@ write SCTLR
> +

To be on the safe side, you could add an isb here. I'm pretty sure it
is immaterial on any ARMv7 core, but hey, I'm paranoid.

With that:

Acked-by: Marc Zyngier <marc.zyngier@arm.com>

Thanks,

	M.

-- 
Jazz is not dead, it just smell funny.

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