Subject: Re: [PATCH updated] libnvdimm/nvdimm/flush: Allow architecture to override the flush barrier

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 30 Jun 2020 10:31:31 +0530

Dan Williams <[email-masked]> writes:

> On Mon, Jun 29, 2020 at 1:29 PM Aneesh Kumar K.V
> <[email-masked]> wrote:
>>
>> Architectures like ppc64 provide persistent memory specific barriers
>> that will ensure that all stores for which the modifications are
>> written to persistent storage by preceding dcbfps and dcbstps
>> instructions have updated persistent storage before any data
>> access or data transfer caused by subsequent instructions is initiated.
>> This is in addition to the ordering done by wmb()
>>
>> Update nvdimm core such that architecture can use barriers other than
>> wmb to ensure all previous writes are architecturally visible for
>> the platform buffer flush.
>>
>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
>> ---
>>  drivers/md/dm-writecache.c   | 2 +-
>>  drivers/nvdimm/region_devs.c | 8 ++++----
>>  include/linux/libnvdimm.h    | 4 ++++
>>  3 files changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
>> index 74f3c506f084..8c6b6dce64e2 100644
>> --- a/drivers/md/dm-writecache.c
>> +++ b/drivers/md/dm-writecache.c
>> @@ -536,7 +536,7 @@ static void ssd_commit_superblock(struct dm_writecache *wc)
>>  static void writecache_commit_flushed(struct dm_writecache *wc, bool wait_for_ios)
>>  {
>>         if (WC_MODE_PMEM(wc))
>> -               wmb();
>> +               arch_pmem_flush_barrier();
>>         else
>>                 ssd_commit_flushed(wc, wait_for_ios);
>>  }
>> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
>> index 4502f9c4708d..b308ad09b63d 100644
>> --- a/drivers/nvdimm/region_devs.c
>> +++ b/drivers/nvdimm/region_devs.c
>> @@ -1206,13 +1206,13 @@ int generic_nvdimm_flush(struct nd_region *nd_region)
>>         idx = this_cpu_add_return(flush_idx, hash_32(current->pid + idx, 8));
>>
>>         /*
>> -        * The first wmb() is needed to 'sfence' all previous writes
>> -        * such that they are architecturally visible for the platform
>> -        * buffer flush.  Note that we've already arranged for pmem
>> +        * The first arch_pmem_flush_barrier() is needed to 'sfence' all
>> +        * previous writes such that they are architecturally visible for
>> +        * the platform buffer flush. Note that we've already arranged for pmem
>>          * writes to avoid the cache via memcpy_flushcache().  The final
>>          * wmb() ensures ordering for the NVDIMM flush write.
>>          */
>> -       wmb();
>> +       arch_pmem_flush_barrier();
>>         for (i = 0; i < nd_region->ndr_mappings; i++)
>>                 if (ndrd_get_flush_wpq(ndrd, i, 0))
>>                         writeq(1, ndrd_get_flush_wpq(ndrd, i, idx));
>> diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
>> index 18da4059be09..66f6c65bd789 100644
>> --- a/include/linux/libnvdimm.h
>> +++ b/include/linux/libnvdimm.h
>> @@ -286,4 +286,8 @@ static inline void arch_invalidate_pmem(void *addr, size_t size)
>>  }
>>  #endif
>>
>> +#ifndef arch_pmem_flush_barrier
>> +#define arch_pmem_flush_barrier() wmb()
>> +#endif
>
> I think it is out of place to define this in libnvdimm.h and it is odd
> to give it such a long name. The other pmem api helpers like
> arch_wb_cache_pmem() and arch_invalidate_pmem() are function calls for
> libnvdimm driver operations, this barrier is just an instruction and
> is closer to wmb() than the pmem api routine.
>
> Since it is a store fence for pmem, so let's just call it pmem_wmb()
> and define the generic version in include/linux/compiler.h. It should
> probably also be documented alongside dma_wmb() in
> Documentation/memory-barriers.txt about why code would use it over
> wmb(), and why a symmetric pmem_rmb() is not needed.

How about the below? I used pmem_barrier() instead of pmem_wmb(). I
guess we wanted this to order() any data access not jus the following
stores to persistent storage? W.r.t why a symmetric pmem_rmb() is not
needed I was not sure how to explain that. Are you suggesting to explain
why a read/load from persistent storage don't want to wait for
pmem_barrier() ?

modified   Documentation/memory-barriers.txt
@@ -1935,6 +1935,16 @@ There are some more advanced barrier functions:
      relaxed I/O accessors and the Documentation/DMA-API.txt file for more
      information on consistent memory.
 
+ (*) pmem_barrier();
+
+     These are for use with persistent memory to esure the ordering of stores
+     to persistent memory region.
+
+     For example, after a non temporal write to persistent storage we use pmem_barrier()
+     to ensures that stores have updated the persistent storage before
+     any data access or data transfer caused by subsequent instructions is initiated.
+
 
 ===============================
 IMPLICIT KERNEL MEMORY BARRIERS
modified   arch/powerpc/include/asm/barrier.h
@@ -97,6 +97,19 @@ do {									\
 #define barrier_nospec()
 #endif /* CONFIG_PPC_BARRIER_NOSPEC */
 
+/*
+ * pmem_barrier() ensures that all stores for which the modification
+ * are written to persistent storage by preceding dcbfps/dcbstps
+ * instructions have updated persistent storage before any data
+ * access or data transfer caused by subsequent instructions is
+ * initiated.
+ */
+#define pmem_barrier pmem_barrier
+static inline void pmem_barrier(void)
+{
+	asm volatile(PPC_PHWSYNC ::: "memory");
+}
+
 #include <asm-generic/barrier.h>
 
 #endif /* _ASM_POWERPC_BARRIER_H */
modified   include/asm-generic/barrier.h
@@ -257,5 +257,16 @@ do {									\
 })
 #endif
 
+/*
+ * pmem_barrier() ensures that all stores for which the modification
+ * are written to persistent storage by preceding instructions have
+ * updated persistent storage before any data  access or data transfer
+ * caused by subsequent instructions is
+ * initiated.
+ */
+#ifndef pmem_barrier
+#define pmem_barrier  wmb()
+#endif
+
 #endif /* !__ASSEMBLY__ */
 #endif /* __ASM_GENERIC_BARRIER_H */