Re: [PATCH v2 5/9] x86, pmem: push fallback handling to arch code

[Williams, Dan J]

On Fri, 2015-08-28 at 15:48 -0600, Toshi Kani wrote:
> On Fri, 2015-08-28 at 14:47 -0700, Dan Williams wrote:
> > On Fri, Aug 28, 2015 at 2:41 PM, Toshi Kani <toshi.kani@xxxxxx> wrote:
> > > On Wed, 2015-08-26 at 21:34 +0000, Williams, Dan J wrote:
> > [..]
> > > > -#define ARCH_MEMREMAP_PMEM MEMREMAP_WB
> > > 
> > > Should it be better to do:
> > > 
> > > #else   /* !CONFIG_ARCH_HAS_PMEM_API */
> > > #define ARCH_MEMREMAP_PMEM MEMREMAP_WT
> > > 
> > > so that you can remove all '#ifdef ARCH_MEMREMAP_PMEM' stuff?
> > 
> > Yeah, that seems like a nice incremental cleanup for memremap_pmem()
> > to just unconditionally use ARCH_MEMREMAP_PMEM, feel free to send it
> > along.
> 
> OK. Will do.
> 

Here's the re-worked patch with Toshi's fixes folded in:

8<-----
Subject: x86, pmem: clarify that ARCH_HAS_PMEM_API implies PMEM mapped WB

From: Dan Williams <dan.j.williams@xxxxxxxxx>

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@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Reviewed-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
[hch: set ARCH_HAS_PMEM_API=n on x86_32]
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
[toshi: x86_32 compile fixes]
Signed-off-by: Toshi Kani <toshi.kani@xxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 arch/x86/Kconfig            |    2 +-
 arch/x86/include/asm/pmem.h |    9 +--------
 drivers/acpi/nfit.c         |    3 ++-
 drivers/nvdimm/pmem.c       |    2 +-
 include/linux/pmem.h        |   36 ++++++++++++++++++++++--------------
 5 files changed, 27 insertions(+), 25 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 03ab6122325a..ef4c6bbb3af1 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -27,7 +27,7 @@ config X86
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_GCOV_PROFILE_ALL
-	select ARCH_HAS_PMEM_API
+	select ARCH_HAS_PMEM_API		if X86_64
 	select ARCH_HAS_MMIO_FLUSH
 	select ARCH_HAS_SG_CHAIN
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
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__ */
diff --git a/drivers/acpi/nfit.c b/drivers/acpi/nfit.c
index 56fff0141636..f61e69fa2ad1 100644
--- a/drivers/acpi/nfit.c
+++ b/drivers/acpi/nfit.c
@@ -20,6 +20,7 @@
 #include <linux/sort.h>
 #include <linux/pmem.h>
 #include <linux/io.h>
+#include <asm/cacheflush.h>
 #include "nfit.h"
 
 /*
@@ -1371,7 +1372,7 @@ static int acpi_nfit_blk_region_enable(struct nvdimm_bus *nvdimm_bus,
 			return -ENOMEM;
 	}
 
-	if (!arch_has_pmem_api() && !nfit_blk->nvdimm_flush)
+	if (!arch_has_wmb_pmem() && !nfit_blk->nvdimm_flush)
 		dev_warn(dev, "unable to guarantee persistence of writes\n");
 
 	if (mmio->line_size == 0)
diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
index 3b5b9cb758b6..20bf122328da 100644
--- a/drivers/nvdimm/pmem.c
+++ b/drivers/nvdimm/pmem.c
@@ -125,7 +125,7 @@ static struct pmem_device *pmem_alloc(struct device *dev,
 
 	pmem->phys_addr = res->start;
 	pmem->size = resource_size(res);
-	if (!arch_has_pmem_api())
+	if (!arch_has_wmb_pmem())
 		dev_warn(dev, "unable to guarantee persistence of writes\n");
 
 	if (!devm_request_mem_region(dev, pmem->phys_addr, pmem->size,
diff --git a/include/linux/pmem.h b/include/linux/pmem.h
index a9d84bf335ee..85f810b33917 100644
--- a/include/linux/pmem.h
+++ b/include/linux/pmem.h
@@ -17,16 +17,23 @@
 #include <linux/uio.h>
 
 #ifdef CONFIG_ARCH_HAS_PMEM_API
+#define ARCH_MEMREMAP_PMEM MEMREMAP_WB
 #include <asm/pmem.h>
 #else
-static inline void arch_wmb_pmem(void)
+#define ARCH_MEMREMAP_PMEM MEMREMAP_WT
+/*
+ * These are simply here to enable compilation, all call sites gate
+ * calling these symbols with arch_has_pmem_api() and redirect to the
+ * implementation in asm/pmem.h.
+ */
+static inline bool __arch_has_wmb_pmem(void)
 {
-	BUG();
+	return false;
 }
 
-static inline bool arch_has_wmb_pmem(void)
+static inline void arch_wmb_pmem(void)
 {
-	return false;
+	BUG();
 }
 
 static inline void arch_memcpy_to_pmem(void __pmem *dst, const void *src,
@@ -53,7 +60,6 @@ static inline void arch_clear_pmem(void __pmem *addr, size_t size)
  * implementations for arch_memcpy_to_pmem(), arch_wmb_pmem(),
  * arch_copy_from_iter_pmem(), arch_clear_pmem() and arch_has_wmb_pmem().
  */
-
 static inline void memcpy_from_pmem(void *dst, void __pmem const *src, size_t size)
 {
 	memcpy(dst, (void __force const *) src, size);
@@ -64,8 +70,13 @@ static inline void memunmap_pmem(struct device *dev, void __pmem *addr)
 	devm_memunmap(dev, (void __force *) addr);
 }
 
+static inline bool arch_has_pmem_api(void)
+{
+	return IS_ENABLED(CONFIG_ARCH_HAS_PMEM_API);
+}
+
 /**
- * arch_has_pmem_api - true if wmb_pmem() ensures durability
+ * arch_has_wmb_pmem - true if wmb_pmem() ensures durability
  *
  * For a given cpu implementation within an architecture it is possible
  * that wmb_pmem() resolves to a nop.  In the case this returns
@@ -73,9 +84,9 @@ static inline void memunmap_pmem(struct device *dev, void __pmem *addr)
  * fall back to a different data consistency model, or otherwise notify
  * the user.
  */
-static inline bool arch_has_pmem_api(void)
+static inline bool arch_has_wmb_pmem(void)
 {
-	return IS_ENABLED(CONFIG_ARCH_HAS_PMEM_API) && arch_has_wmb_pmem();
+	return arch_has_pmem_api() && __arch_has_wmb_pmem();
 }
 
 /*
@@ -120,13 +131,8 @@ static inline void default_clear_pmem(void __pmem *addr, size_t size)
 static inline void __pmem *memremap_pmem(struct device *dev,
 		resource_size_t offset, unsigned long size)
 {
-#ifdef ARCH_MEMREMAP_PMEM
 	return (void __pmem *) devm_memremap(dev, offset, size,
 			ARCH_MEMREMAP_PMEM);
-#else
-	return (void __pmem *) devm_memremap(dev, offset, size,
-			MEMREMAP_WT);
-#endif
 }
 
 /**
@@ -158,8 +164,10 @@ static inline void memcpy_to_pmem(void __pmem *dst, const void *src, size_t n)
  */
 static inline void wmb_pmem(void)
 {
-	if (arch_has_pmem_api())
+	if (arch_has_wmb_pmem())
 		arch_wmb_pmem();
+	else
+		wmb();
 }
 
 /**