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SubjectRe: [RFC PATCH 3/7] arm64: mm: use nGnRnE instead of nGnRE on Apple processors
On 2021-01-21 11:27, Will Deacon wrote:
> 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.

Would that imply another round-trip into the idmap, much like we do
when we switch to non-global mappings? Or do you expect that we can
change
the MAIR with live mappings?

M.
--
Jazz is not dead. It just smells funny...

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