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

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Message-Id: <17579DC5-13AF-48E1-9B72-91B14AB485FA@caramail.com>
Date:   Wed, 20 Jan 2021 19:06:48 +0100
From:   Mohamed Mediouni <mohamed.mediouni@...amail.com>
To:     Alexander Graf <graf@...zon.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        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 Jan 2021, at 17:47, Alexander Graf <graf@...zon.com> wrote:
> 
> 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
> 
Not a particular reason, as we explicitly check for the implementer ID. However,
without CONFIG_ARCH_APPLE, other parts of the support for Apple CPUs
will not be available anyway.
>> +
>> +/*
>> + * 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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