Re: [PATCH v2] arm64: errata: Handle Apple WFI State Loss

[Sven Peter <sven@kernel.org>]

On 15.06.26 17:02, Will Deacon wrote:
> On Mon, Jun 15, 2026 at 02:21:36PM +0200, Yureka Lilian wrote:
>> Apple Silicon CPUs can lose register state in WFI, leading to crashes
>> in the idle loop early in the boot process.
>> This applies to any previous Apple Silicon CPUs too, but is worked
>> around by configuring the WFI mode in SYS_IMP_APL_CYC_OVRD sysreg
>> during m1n1's chickens setup.
>> This workaround no longer exists since M4.
>>
>> Add a workaround capability for replacing wfi and wfit with nop, and
>> an erratum to enable it on the affected CPUs if the workaround using the
>> sysreg is not already applied. Leave the decision whether the sysreg
>> workaround can be used up to the earlier parts of the boot chain which
>> already configure the Apple Silicon chicken bits.
>>
>> This alternative has to be applied in early boot, since otherwise some
>> cores might enter the idle loop before apply_alternatives_all() is run.
>>
>> Reviewed-by: Sasha Finkelstein <k@chaosmail.tech>
>> Signed-off-by: Yureka Lilian <yureka@cyberchaos.dev>
>> ---
>> Changes since v1:
>> Restricted the erratum to EL2 only, since in EL1 we'd expect the
>> hypervisor to trap WFI and handle the erratum.
>>
>> Tested on M4 and M4 Pro (which now sometimes nondeterministically
>> crash later during boot).
>> Successfully booted on M3 Max with the SYS_IMP_APL_CYC_OVRD
>> workaround disabled in the bootloader, as well as A18 Pro (which,
>> like M4 / M4 Pro, doesn't have SYS_IMP_APL_CYC_OVRD).
>>
>> There is probably a better place for the SYS_IMP_APL_CYC_OVRD
>> defines, which I currently put in the middle of cpu_errata.c, but I
>> wouldn't know where.
>> ---
>>   arch/arm64/Kconfig               | 12 ++++++++++++
>>   arch/arm64/include/asm/barrier.h | 19 ++++++++++++++++---
>>   arch/arm64/kernel/cpu_errata.c   | 21 +++++++++++++++++++++
>>   arch/arm64/tools/cpucaps         |  1 +
>>   4 files changed, 50 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index b3afe0688919..8c8ff069856f 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -453,6 +453,18 @@ config AMPERE_ERRATUM_AC04_CPU_23
>>   
>>   	  If unsure, say Y.
>>   
>> +config APPLE_ERRATUM_WFI_STATE
>> +	bool "Apple Silicon: WFI loses state"
>> +	default y
>> +	help
>> +	  This option adds an alternative code sequence to work around some
>> +	  Apple Silicon CPUs losing register state during wfi and wfit
>> +	  instructions.
>> +
>> +	  As a workaround, the wfi and wfit instructions are replaced with nop
>> +	  operations via the alternative framework if an affected CPU is
>> +	  detected.
>> +
>>   config ARM64_WORKAROUND_CLEAN_CACHE
>>   	bool
>>   
>> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
>> index 9495c4441a46..f72eddc7c434 100644
>> --- a/arch/arm64/include/asm/barrier.h
>> +++ b/arch/arm64/include/asm/barrier.h
>> @@ -20,9 +20,22 @@
>>   #define wfe()		asm volatile("wfe" : : : "memory")
>>   #define wfet(val)	asm volatile("msr s0_3_c1_c0_0, %0"	\
>>   				     : : "r" (val) : "memory")
>> -#define wfi()		asm volatile("wfi" : : : "memory")
>> -#define wfit(val)	asm volatile("msr s0_3_c1_c0_1, %0"	\
>> -				     : : "r" (val) : "memory")
>> +#define wfi()							\
>> +	do {							\
>> +		asm volatile(					\
>> +		ALTERNATIVE("wfi",				\
>> +			    "nop",				\
>> +			    ARM64_WORKAROUND_WFI_STATE)		\
>> +		: : : "memory");				\
>> +	} while (0)
>> +#define wfit(val)						\
>> +	do {							\
>> +		asm volatile(					\
>> +		ALTERNATIVE("msr s0_3_c1_c0_1, %0",		\
>> +			    "nop",				\
>> +			    ARM64_WORKAROUND_WFI_STATE)		\
>> +		: : "r" (val) : "memory");			\
>> +	} while (0)
> 
> How can you guarantee that we don't run one of these prior to patching?
> 
> I wonder if we're better off doing something like x86 and having an "idle="
> cmdline option. which could choose between e.g. nop, wfe, wfi and yield, for
> example? In fact, would wfe be a better choice than nop for you?

I think that should also solve the issue of detecting if we're running 
on bare metal and need this or if there's something that mitigates the 
issue (e.g. M1-M3 where we can still enable that chicken bit or a 
hypervisor that just traps wfi): Just let the bootloader decide and 
inject that cmdline option.


Best,


Sven