Re: [RFC PATCH 3/7] arm64: mm: use nGnRnE instead of nGnRE on Apple processors

[Will Deacon <will@kernel.org>]

On Wed, Jan 20, 2021 at 02:27:13PM +0100, Mohamed Mediouni wrote:
> Use nGnRnE instead of nGnRE on Apple SoCs to workaround a serious hardware quirk.
> 
> On Apple processors, writes using the nGnRE device memory type get dropped in flight,
> getting to nowhere.
> 
> Signed-off-by: Stan Skowronek <stan@corellium.com>
> Signed-off-by: Mohamed Mediouni <mohamed.mediouni@caramail.com>
> ---
>  arch/arm64/mm/proc.S | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index 1f7ee8c8b7b8..06436916f137 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -51,6 +51,25 @@
>  #define TCR_KASAN_HW_FLAGS 0
>  #endif
> 
> +#ifdef CONFIG_ARCH_APPLE
> +
> +/*
> + * Apple cores appear to black-hole writes done with nGnRE.
> + * We settled on a work-around that uses MAIR vs changing every single user of
> + * nGnRE across the arm64 code.
> + */
> +
> +#define MAIR_EL1_SET_APPLE						\
> +	(MAIR_ATTRIDX(MAIR_ATTR_DEVICE_nGnRnE, MT_DEVICE_nGnRnE) |	\
> +	 MAIR_ATTRIDX(MAIR_ATTR_DEVICE_nGnRnE, MT_DEVICE_nGnRE) |	\
> +	 MAIR_ATTRIDX(MAIR_ATTR_DEVICE_GRE, MT_DEVICE_GRE) |		\
> +	 MAIR_ATTRIDX(MAIR_ATTR_NORMAL_NC, MT_NORMAL_NC) |		\
> +	 MAIR_ATTRIDX(MAIR_ATTR_NORMAL, MT_NORMAL) |			\
> +	 MAIR_ATTRIDX(MAIR_ATTR_NORMAL_WT, MT_NORMAL_WT) |		\
> +	 MAIR_ATTRIDX(MAIR_ATTR_NORMAL, MT_NORMAL_TAGGED))
> +
> +#endif
> +
>  /*
>   * Default MAIR_EL1. MT_NORMAL_TAGGED is initially mapped as Normal memory and
>   * changed during __cpu_setup to Normal Tagged if the system supports MTE.
> @@ -432,6 +451,13 @@ SYM_FUNC_START(__cpu_setup)
>  	 * Memory region attributes
>  	 */
>  	mov_q	x5, MAIR_EL1_SET
> +#ifdef CONFIG_ARCH_APPLE
> +	mrs	x0, MIDR_EL1
> +	lsr	w0, w0, #24
> +	mov_q	x1, MAIR_EL1_SET_APPLE
> +	cmp	x0, #0x61			// 0x61 = Implementer: Apple
> +	csel	x5, x1, x5, eq

Why does this need to be done so early? It would be a lot cleaner if we
could detect this in a similar fashion to other errata and update the MAIR
appropriately. If that's not possible because of early IO mappings (which
ones?), then we could instead initialise to nGnRnE unconditionally, but
relax it to nGnRE if we detect that we _don't_ have the erratum.

Will