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

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Message-ID: <ce5692a9-e716-514a-f196-b0645bb03eb8@amazon.com>
Date:   Wed, 20 Jan 2021 17:47:33 +0100
From:   Alexander Graf <graf@...zon.com>
To:     Mohamed Mediouni <mohamed.mediouni@...amail.com>,
        <linux-arm-kernel@...ts.infradead.org>
CC:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        "Hector Martin" <marcan@...can.st>, <linux-kernel@...r.kernel.org>,
        Stan Skowronek <stan@...ellium.com>
Subject: Re: [RFC PATCH 3/7] arm64: mm: use nGnRnE instead of nGnRE on Apple
 processors

On 20.01.21 14:27, 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@...ellium.com>
> Signed-off-by: Mohamed Mediouni <mohamed.mediouni@...amail.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

Is there any particular reason for this #ifdef?


Alex

> +
> +/*
> + * 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
> +#endif
>   #ifdef CONFIG_ARM64_MTE
>   	mte_tcr	.req	x20
> 
> --
> 2.29.2
> 




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