x86, pmem: clarify that ARCH_HAS_PMEM_API implies PMEM mapped WB

Given that a write-back (WB) mapping plus non-temporal stores is
expected to be the most efficient way to access PMEM, update the
definition of ARCH_HAS_PMEM_API to imply arch support for
WB-mapped-PMEM.  This is needed as a pre-requisite for adding PMEM to
the direct map and mapping it with struct page.

The above clarification for X86_64 means that memcpy_to_pmem() is
permitted to use the non-temporal arch_memcpy_to_pmem() rather than
needlessly fall back to default_memcpy_to_pmem() when the pcommit
instruction is not available.  When arch_memcpy_to_pmem() is not
guaranteed to flush writes out of cache, i.e. on older X86_32
implementations where non-temporal stores may just dirty cache,
ARCH_HAS_PMEM_API is simply disabled.

The default fall back for persistent memory handling remains.  Namely,
map it with the WT (write-through) cache-type and hope for the best.

arch_has_pmem_api() is updated to only indicate whether the arch
provides the proper helpers to meet the minimum "writes are visible
outside the cache hierarchy after memcpy_to_pmem() + wmb_pmem()".  Code
that cares whether wmb_pmem() actually flushes writes to pmem must now
call arch_has_wmb_pmem() directly.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
[hch: set ARCH_HAS_PMEM_API=n on x86_32]
Reviewed-by: Christoph Hellwig <hch@lst.de>
[toshi: x86_32 compile fixes]
Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 1 insertions, 8 deletions

diff --git a/arch/x86/include/asm/pmem.h b/arch/x86/include/asm/pmem.h
index bb026c5adf8a..d8ce3ec816ab 100644
--- a/arch/x86/include/asm/pmem.h
+++ b/arch/x86/include/asm/pmem.h
@@ -18,8 +18,6 @@
 #include <asm/cpufeature.h>
 #include <asm/special_insns.h>
 
-#define ARCH_MEMREMAP_PMEM MEMREMAP_WB
-
 #ifdef CONFIG_ARCH_HAS_PMEM_API
 /**
  * arch_memcpy_to_pmem - copy data to persistent memory
@@ -143,18 +141,13 @@ static inline void arch_clear_pmem(void __pmem *addr, size_t size)
         __arch_wb_cache_pmem(vaddr, size);
 }
 
-static inline bool arch_has_wmb_pmem(void)
+static inline bool __arch_has_wmb_pmem(void)
 {
-#ifdef CONFIG_X86_64
         /*
          * We require that wmb() be an 'sfence', that is only guaranteed on
          * 64-bit builds
          */
         return static_cpu_has(X86_FEATURE_PCOMMIT);
-#else
-        return false;
-#endif
 }
 #endif /* CONFIG_ARCH_HAS_PMEM_API */
-
 #endif /* __ASM_X86_PMEM_H__ */