Re: [PATCH v5] x86/mce: retrieve poison range from hardware

[Jane Chu]

On 8/1/2022 9:44 AM, Dan Williams wrote:
> Jane Chu wrote:
>> With Commit 7917f9cdb503 ("acpi/nfit: rely on mce->misc to determine
>> poison granularity") that changed nfit_handle_mce() callback to report
>> badrange according to 1ULL << MCI_MISC_ADDR_LSB(mce->misc), it's been
>> discovered that the mce->misc LSB field is 0x1000 bytes, hence injecting
>> 2 back-to-back poisons and the driver ends up logging 8 badblocks,
>> because 0x1000 bytes is 8 512-byte.
>>
>> Dan Williams noticed that apei_mce_report_mem_error() hardcode
>> the LSB field to PAGE_SHIFT instead of consulting the input
>> struct cper_sec_mem_err record.  So change to rely on hardware whenever
>> support is available.
>>
>> Link: https://lore.kernel.org/r/7ed50fd8-521e-cade-77b1-738b8bfb8502@xxxxxxxxxx
>>
>> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> Signed-off-by: Jane Chu <jane.chu@xxxxxxxxxx>
>> ---
>>   arch/x86/kernel/cpu/mce/apei.c | 14 +++++++++++++-
>>   1 file changed, 13 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/cpu/mce/apei.c b/arch/x86/kernel/cpu/mce/apei.c
>> index 717192915f28..2c7ea0ba9dd7 100644
>> --- a/arch/x86/kernel/cpu/mce/apei.c
>> +++ b/arch/x86/kernel/cpu/mce/apei.c
>> @@ -29,15 +29,27 @@
>>   void apei_mce_report_mem_error(int severity, struct cper_sec_mem_err *mem_err)
>>   {
>>   	struct mce m;
>> +	int lsb = PAGE_SHIFT;
>>   
>>   	if (!(mem_err->validation_bits & CPER_MEM_VALID_PA))
>>   		return;
>>   
>> +	/*
>> +	 * Even if the ->validation_bits are set for address mask,
>> +	 * to be extra safe, check and reject an error radius '0',
>> +	 * and fallback to the default page size.
>> +	 */
>> +	if (mem_err->validation_bits & CPER_MEM_VALID_PA_MASK) {
>> +		lsb = __ffs64(mem_err->physical_addr_mask);
>> +		if (lsb == 1)
> 
> This was the reason I recommended hweight64 and min_not_zero() as
> hweight64 does not have the undefined behavior. However, an even better
> option is to just do:
> 
>      find_first_bit(&mem_err->physical_addr_mask, PAGE_SHIFT)
> 
> ...as that trims the result to the PAGE_SHIFT max and handles the zero
> case.

Thanks Dan!  However it looks like find_first_bit() could call into 
__ffs(x) which has the same limitation as __ffs64(x), as Tony pointed out.

I'll post v6 shortly.

thanks,
-jane