[PATCH] x86: Add kerneldoc for pcommit_sfence()

[Ross Zwisler]

Add kerneldoc comments for pcommit_sfence() describing the purpose of
the pcommit instruction and demonstrating the usage of that instruction.

Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Cc: H Peter Anvin <h.peter.anvin@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
---
 arch/x86/include/asm/special_insns.h | 37 ++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index aeb4666e0c0a..1ae81757c05b 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -215,6 +215,43 @@ static inline void clwb(volatile void *__p)
 		: [pax] "a" (p));
 }
 
+/**
+ * pcommit_sfence() - persistent commit and fence
+ *
+ * The pcommit instruction ensures that data that has been flushed from the
+ * processor's cache hierarchy with clwb, clflushopt or clflush is accepted to
+ * memory and is durable on the DIMM.  The primary use case for this is
+ * persistent memory.
+ *
+ * This function shows how to properly use clwb/clflushopt/clflush and pcommit
+ * with appropriate fencing:
+ *
+ * void flush_and_commit_buffer(void *vaddr, unsigned int size)
+ * {
+ *         unsigned long clflush_mask = boot_cpu_data.x86_clflush_size - 1;
+ *         char *vend = (char *)vaddr + size;
+ *         char *p;
+ *
+ *         for (p = (char *)((unsigned long)vaddr & ~clflush_mask);
+ *              p < vend; p += boot_cpu_data.x86_clflush_size)
+ *                 clwb(p);
+ *
+ *         // sfence to order clwb/clflushopt/clflush cache flushes
+ *         // mfence via mb() also works
+ *         wmb();
+ *
+ *         // pcommit and the required sfence for ordering
+ *         pcommit_sfence();
+ * }
+ *
+ * After this function completes the data pointed to by vaddr is has been
+ * accepted to memory and will be durable if the vaddr points to persistent
+ * memory.
+ *
+ * Pcommit must always be ordered by an mfence or sfence, so to help simplify
+ * things we include both the pcommit and the required sfence in the
+ * alternatives generated by pcommit_sfence().
+ */
 static inline void pcommit_sfence(void)
 {
 	alternative(ASM_NOP7,
-- 
1.9.3

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