Dan Williams [off-list ref] writes:

On Mon, Jun 29, 2020 at 1:29 PM Aneesh Kumar K.V
[off-list ref] wrote:

quoted

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 <redacted>
---
 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 */