Subject: Re: [PATCH v6 6/8] powerpc/pmem: Avoid the barrier in flush routines

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 30 Jun 2020 02:10:15 +0530

Michal Suchánek <[email-masked]> writes:

> Hello,
>
> On Mon, Jun 29, 2020 at 07:27:20PM +0530, Aneesh Kumar K.V wrote:
>> nvdimm expect the flush routines to just mark the cache clean. The barrier
>> that mark the store globally visible is done in nvdimm_flush().
>> 
>> Update the papr_scm driver to a simplified nvdim_flush callback that do
>> only the required barrier.
>> 
>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
>> ---
>>  arch/powerpc/lib/pmem.c                   |  6 ------
>>  arch/powerpc/platforms/pseries/papr_scm.c | 13 +++++++++++++
>>  2 files changed, 13 insertions(+), 6 deletions(-)
>> 
>> diff --git a/arch/powerpc/lib/pmem.c b/arch/powerpc/lib/pmem.c
>> index 5a61aaeb6930..21210fa676e5 100644
>> --- a/arch/powerpc/lib/pmem.c
>> +++ b/arch/powerpc/lib/pmem.c
>> @@ -19,9 +19,6 @@ static inline void __clean_pmem_range(unsigned long start, unsigned long stop)
>>  
>>  	for (i = 0; i < size >> shift; i++, addr += bytes)
>>  		asm volatile(PPC_DCBSTPS(%0, %1): :"i"(0), "r"(addr): "memory");
>> -
>> -
>> -	asm volatile(PPC_PHWSYNC ::: "memory");
>>  }
>>  
>>  static inline void __flush_pmem_range(unsigned long start, unsigned long stop)
>> @@ -34,9 +31,6 @@ static inline void __flush_pmem_range(unsigned long start, unsigned long stop)
>>  
>>  	for (i = 0; i < size >> shift; i++, addr += bytes)
>>  		asm volatile(PPC_DCBFPS(%0, %1): :"i"(0), "r"(addr): "memory");
>> -
>> -
>> -	asm volatile(PPC_PHWSYNC ::: "memory");
>>  }
>>  
>>  static inline void clean_pmem_range(unsigned long start, unsigned long stop)
>> diff --git a/arch/powerpc/platforms/pseries/papr_scm.c b/arch/powerpc/platforms/pseries/papr_scm.c
>> index 9c569078a09f..9a9a0766f8b6 100644
>> --- a/arch/powerpc/platforms/pseries/papr_scm.c
>> +++ b/arch/powerpc/platforms/pseries/papr_scm.c
>> @@ -630,6 +630,18 @@ static int papr_scm_ndctl(struct nvdimm_bus_descriptor *nd_desc,
>>  
>>  	return 0;
>>  }
>> +/*
>> + * We have made sure the pmem writes are done such that before calling this
>> + * all the caches are flushed/clean. We use dcbf/dcbfps to ensure this. Here
>> + * we just need to add the necessary barrier to make sure the above flushes
>> + * are have updated persistent storage before any data access or data transfer
>> + * caused by subsequent instructions is initiated.
>> + */
>> +static int papr_scm_flush_sync(struct nd_region *nd_region, struct bio *bio)
>> +{
>> +	arch_pmem_flush_barrier();
>> +	return 0;
>> +}
>>  
>>  static ssize_t flags_show(struct device *dev,
>>  			  struct device_attribute *attr, char *buf)
>> @@ -743,6 +755,7 @@ static int papr_scm_nvdimm_init(struct papr_scm_priv *p)
>>  	ndr_desc.mapping = &mapping;
>>  	ndr_desc.num_mappings = 1;
>>  	ndr_desc.nd_set = &p->nd_set;
>> +	ndr_desc.flush = papr_scm_flush_sync;
>
> AFAICT currently the only device that implements flush is virtio_pmem.
> How does the nfit driver get away without implementing flush?

generic_nvdimm_flush does the required barrier for nfit. The reason for
adding ndr_desc.flush call back for papr_scm was to avoid the usage
of iomem based deep flushing (ndr_region_data.flush_wpq) which is not
supported by papr_scm.

BTW we do return NULL for ndrd_get_flush_wpq() on power. So the upstream
code also does the same thing, but in a different way.

> Also the flush takes arguments that are completely unused but a user of
> the pmem region must assume they are used, and call flush() on the
> region rather than arch_pmem_flush_barrier() directly.

The bio argument can help a pmem driver to do range based flushing in
case of pmem_make_request. If bio is null then we must assume a full
device flush. 

>This may not
> work well with md as discussed with earlier iteration of the patchest.
>

dm-writecache needs some major changes to work with asynchronous pmem
devices. 

-aneesh