Re: [PATCH] powerpc/papr_scm: Reduce error severity if nvdimm stats inaccessible

[Vaibhav Jain <vaibhav@linux.ibm.com>]

Thanks for looking into this Dan,

Dan Williams <dan.j.williams@intel.com> writes:

> On Wed, Apr 14, 2021 at 5:40 AM Vaibhav Jain <vaibhav@linux.ibm.com> wrote:
>>
>> Currently drc_pmem_qeury_stats() generates a dev_err in case
>> "Enable Performance Information Collection" feature is disabled from
>> HMC. The error is of the form below:
>>
>> papr_scm ibm,persistent-memory:ibm,pmemory@44104001: Failed to query
>>          performance stats, Err:-10
>>
>> This error message confuses users as it implies a possible problem
>> with the nvdimm even though its due to a disabled feature.
>>
>> So we fix this by explicitly handling the H_AUTHORITY error from the
>> H_SCM_PERFORMANCE_STATS hcall and generating a warning instead of an
>> error, saying that "Performance stats in-accessible".
>>
>> Fixes: 2d02bf835e57('powerpc/papr_scm: Fetch nvdimm performance stats from PHYP')
>> Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
>> ---
>>  arch/powerpc/platforms/pseries/papr_scm.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/powerpc/platforms/pseries/papr_scm.c b/arch/powerpc/platforms/pseries/papr_scm.c
>> index 835163f54244..9216424f8be3 100644
>> --- a/arch/powerpc/platforms/pseries/papr_scm.c
>> +++ b/arch/powerpc/platforms/pseries/papr_scm.c
>> @@ -277,6 +277,9 @@ static ssize_t drc_pmem_query_stats(struct papr_scm_priv *p,
>>                 dev_err(&p->pdev->dev,
>>                         "Unknown performance stats, Err:0x%016lX\n", ret[0]);
>>                 return -ENOENT;
>> +       } else if (rc == H_AUTHORITY) {
>> +               dev_warn(&p->pdev->dev, "Performance stats in-accessible");
>> +               return -EPERM;
>
> So userspace can spam the kernel log? Why is kernel log message needed
> at all? EPERM told the caller what happened.
Currently this error message is only reported during probe of the
nvdimm. So userspace cannot directly spam kernel log.

The callsite for this function in papr_scm_nvdimm_init() doesnt handle
specific error codes. Instead in case of an error it only reports that
"Dimm performance stats are unavailable". The log message just
preceeding that mentions the real cause of failure. Thats why just
returning -EPERM wont be usefui.

Alternatively I can update papr_scm_nvdimm_init() to report the error
code returned from drc_pmem_query_stats().

-- 
Cheers
~ Vaibhav
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