Re: [PATCH v9 1/2] ACPI: APEI: set memory failure flags as MF_ACTION_REQUIRED on synchronous events

[Shuai Xue <xueshuai@linux.alibaba.com>]

On 2023/12/1 01:39, James Morse wrote:
> Hi Shuai,
> 
> On 07/10/2023 08:28, Shuai Xue wrote:
>> There are two major types of uncorrected recoverable (UCR) errors :
> 
> Is UCR a well known x86 acronym? It's best to just spell this out each time,
> there is enough jargon in this area already.

Quite agreed, will replace the commit log with "uncorrected recoverable error".

> 
>>
>> - Action Required (AR): The error is detected and the processor already
>>   consumes the memory. OS requires to take action (for example, offline
>>   failure page/kill failure thread) to recover this uncorrectable error.
>>
>> - Action Optional (AO): The error is detected out of processor execution
>>   context. Some data in the memory are corrupted. But the data have not
>>   been consumed. OS is optional to take action to recover this
>>   uncorrectable error.
> 
> As elsewhere, please don't think of errors as 'action required', this is how
> things get reported to user-space. Action-required for one thread may be
> action-optional for another that has the same page mapped - its really not a
> property of the error.
> It would be better to describe this as synchronous and asynchronous, or in-band
> and out-of-band.

Thank you for explanation. I will change to "synchronous and asynchronous".

> 
> 
>> The essential difference between AR and AO errors is that AR is a
>> synchronous event, while AO is an asynchronous event. The hardware will
>> signal a synchronous exception (Machine Check Exception on X86 and
>> Synchronous External Abort on Arm64) when an error is detected and the
>> memory access has been architecturally executed.
> 
>> When APEI firmware first is enabled, a platform may describe one error
>> source for the handling of synchronous errors (e.g. MCE or SEA notification
>> ), or for handling asynchronous errors (e.g. SCI or External Interrupt
>> notification). In other words, we can distinguish synchronous errors by
>> APEI notification. For AR errors, kernel will kill current process
>> accessing the poisoned page by sending SIGBUS with BUS_MCEERR_AR. In
>> addition, for AO errors, kernel will notify the process who owns the
>> poisoned page by sending SIGBUS with BUS_MCEERR_AO in early kill mode.
>> However, the GHES driver always sets mf_flags to 0 so that all UCR errors
>> are handled as AO errors in memory failure.
> 
> To make this easier to read:
>  UCR and AR -> synchronous
>  AO -> asynchronous
> 

Will do that.

> 
>> To this end, set memory failure flags as MF_ACTION_REQUIRED on synchronous
>> events.
> 
>> Fixes: ba61ca4aab47 ("ACPI, APEI, GHES: Add hardware memory error recovery support")'
> 
> Erm, this predates arm64 support, and what you have here doesn't change the behaviour on x86.
> 
> You can blame 7f17b4a121d0d50 ("ACPI: APEI: Kick the memory_failure() queue for
> synchronous errors"), which should have covered this.

Do you mean just drop the "Fixes" tags?

> 
>> diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
>> index ef59d6ea16da..88178aa6222d 100644
>> --- a/drivers/acpi/apei/ghes.c
>> +++ b/drivers/acpi/apei/ghes.c
>> @@ -101,6 +101,20 @@ static inline bool is_hest_type_generic_v2(struct ghes *ghes)
>>  	return ghes->generic->header.type == ACPI_HEST_TYPE_GENERIC_ERROR_V2;
>>  }
>>  
>> +/*
>> + * A platform may describe one error source for the handling of synchronous
>> + * errors (e.g. MCE or SEA), or for handling asynchronous errors (e.g. SCI
>> + * or External Interrupt). On x86, the HEST notifications are always
>> + * asynchronous, so only SEA on ARM is delivered as a synchronous
>> + * notification.
>> + */
>> +static inline bool is_hest_sync_notify(struct ghes *ghes)
>> +{
>> +	u8 notify_type = ghes->generic->notify.type;
>> +
>> +	return notify_type == ACPI_HEST_NOTIFY_SEA;
>> +}
> 
> and as you had in earlier versions, sometimes SDEI.
> SDEI can report by synchronous and asynchronous errors, I wouldn't too surprised if the
> hardware NMI can be used for the same. It would be good to chase up having a hint of this
> in the CPER records and pass that in here as a hint.> 
> Unfortunately, its not safe to assume either way for SDEI.

For SDEI notification, only x0-x17 has preserved by firmware.  As SDEI
TRM[1] describes "the dispatcher can simulate an exception-like entry into
the client, **with the client providing an additional asynchronous entry
point similar to an interrupt entry point**".  The client (kernel) lacks
complete synchronous context, e.g. system register (ELR, ESR, etc). So I
think SDEI notification should not be used for asynchronous error, can you
help to confirm this?

For NMI notification, as far as I know, AArch64 (aka arm64 in the Linux
tree) does not provide architected NMIs.

> 
> Reviewed-by: James Morse <james.morse@arm.com>
> 

Thank you for valuable comments.

Best Regards,
Shuai

[1] https://developer.arm.com/documentation/den0054/latest/