[PATCH v4 6/6] arch, x86: pmem api for ensuring durability of persistent memory updates

[Dan Williams]

From: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>

Based on an original patch by Ross Zwisler [1].

Writes to persistent memory have the potential to be posted to cpu
cache, cpu write buffers, and platform write buffers (memory controller)
before being committed to persistent media.  Provide apis,
memcpy_to_pmem(), sync_pmem(), and memremap_pmem(), to write data to
pmem and assert that it is durable in PMEM (a persistent linear address
range).  A '__pmem' attribute is added so sparse can track proper usage
of pointers to pmem.

[1]: https://lists.01.org/pipermail/linux-nvdimm/2015-May/000932.html

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
[djbw: various reworks]
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 arch/x86/Kconfig                  |    1 
 arch/x86/include/asm/cacheflush.h |   36 +++++++++++++
 arch/x86/include/asm/io.h         |    6 ++
 drivers/block/pmem.c              |   75 +++++++++++++++++++++++++--
 include/linux/compiler.h          |    2 +
 include/linux/pmem.h              |  102 +++++++++++++++++++++++++++++++++++++
 lib/Kconfig                       |    3 +
 7 files changed, 218 insertions(+), 7 deletions(-)
 create mode 100644 include/linux/pmem.h

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f16caf7eac27..5dfb8f31ac48 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -28,6 +28,7 @@ config X86
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_GCOV_PROFILE_ALL
 	select ARCH_HAS_MEMREMAP
+	select ARCH_HAS_PMEM_API
 	select ARCH_HAS_SG_CHAIN
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
 	select ARCH_MIGHT_HAVE_ACPI_PDC		if ACPI
diff --git a/arch/x86/include/asm/cacheflush.h b/arch/x86/include/asm/cacheflush.h
index b6f7457d12e4..4d896487382c 100644
--- a/arch/x86/include/asm/cacheflush.h
+++ b/arch/x86/include/asm/cacheflush.h
@@ -4,6 +4,7 @@
 /* Caches aren't brain-dead on the intel. */
 #include <asm-generic/cacheflush.h>
 #include <asm/special_insns.h>
+#include <asm/uaccess.h>
 
 /*
  * The set_memory_* API can be used to change various attributes of a virtual
@@ -108,4 +109,39 @@ static inline int rodata_test(void)
 }
 #endif
 
+#ifdef ARCH_HAS_NOCACHE_UACCESS
+static inline void arch_memcpy_to_pmem(void __pmem *dst, const void *src, size_t n)
+{
+	/*
+	 * We are copying between two kernel buffers, if
+	 * __copy_from_user_inatomic_nocache() returns an error (page
+	 * fault) we would have already taken an unhandled fault before
+	 * the BUG_ON.  The BUG_ON is simply here to satisfy
+	 * __must_check and allow reuse of the common non-temporal store
+	 * implementation for memcpy_to_pmem().
+	 */
+	BUG_ON(__copy_from_user_inatomic_nocache((void __force *) dst,
+				(void __user *) src, n));
+}
+
+static inline void arch_sync_pmem(void)
+{
+	wmb();
+	pcommit_sfence();
+}
+
+static inline bool __arch_has_sync_pmem(void)
+{
+	return boot_cpu_has(X86_FEATURE_PCOMMIT);
+}
+#else /* ARCH_HAS_NOCACHE_UACCESS i.e. ARCH=um */
+extern void arch_memcpy_to_pmem(void __pmem *dst, const void *src, size_t n);
+extern void arch_sync_pmem(void);
+
+static inline bool __arch_has_sync_pmem(void)
+{
+	return false;
+}
+#endif
+
 #endif /* _ASM_X86_CACHEFLUSH_H */
diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index e9d6691ec4c5..0a494ac22a8e 100644
--- a/arch/x86/include/asm/io.h
+++ b/arch/x86/include/asm/io.h
@@ -249,6 +249,12 @@ static inline void flush_write_buffers(void)
 #endif
 }
 
+static inline void __pmem *arch_memremap_pmem(resource_size_t offset,
+	unsigned long size)
+{
+	return (void __force __pmem *) ioremap_cache(offset, size);
+}
+
 #endif /* __KERNEL__ */
 
 extern void native_io_delay(void);
diff --git a/drivers/block/pmem.c b/drivers/block/pmem.c
index b00b97314b57..81090f61b8b1 100644
--- a/drivers/block/pmem.c
+++ b/drivers/block/pmem.c
@@ -23,23 +23,79 @@
 #include <linux/module.h>
 #include <linux/moduleparam.h>
 #include <linux/slab.h>
+#include <linux/pmem.h>
 #include <linux/io.h>
 
 #define PMEM_MINORS		16
 
+struct pmem_ops {
+	void __pmem *(*remap)(resource_size_t offset, unsigned long size);
+	void (*copy)(void __pmem *dst, const void *src, size_t size);
+	void (*sync)(void);
+};
+
 struct pmem_device {
 	struct request_queue	*pmem_queue;
 	struct gendisk		*pmem_disk;
 
 	/* One contiguous memory region per device */
 	phys_addr_t		phys_addr;
-	void			*virt_addr;
+	void __pmem		*virt_addr;
 	size_t			size;
+	struct pmem_ops		ops;
 };
 
 static int pmem_major;
 static atomic_t pmem_index;
 
+static void default_sync_pmem(void)
+{
+	wmb();
+}
+
+static void default_memcpy_to_pmem(void __pmem *dst, const void *src, size_t size)
+{
+	memcpy((void __force *) dst, src, size);
+}
+
+static void __pmem *default_memremap_pmem(resource_size_t offset, unsigned long size)
+{
+	return (void __pmem *)memremap_wt(offset, size);
+}
+
+static void pmem_ops_default_init(struct pmem_device *pmem)
+{
+	/*
+	 * These defaults seek to offer decent performance and minimize
+	 * the window between i/o completion and writes being durable on
+	 * media.  However, it is undefined / architecture specific
+	 * whether default_memcpy_to_pmem + default_pmem_sync is
+	 * sufficient for making data durable relative to i/o
+	 * completion.
+	 */
+	pmem->ops.remap = default_memremap_pmem;
+	pmem->ops.copy = default_memcpy_to_pmem;
+	pmem->ops.sync = default_sync_pmem;
+}
+
+static bool pmem_ops_init(struct pmem_device *pmem)
+{
+	if (IS_ENABLED(CONFIG_ARCH_HAS_PMEM_API) &&
+			arch_has_sync_pmem()) {
+		/*
+		 * This arch + cpu guarantees that bio_endio() == data
+		 * durable on media.
+		 */
+		pmem->ops.remap = memremap_pmem;
+		pmem->ops.copy = memcpy_to_pmem;
+		pmem->ops.sync = sync_pmem;
+		return true;
+	}
+
+	pmem_ops_default_init(pmem);
+	return false;
+}
+
 static void pmem_do_bvec(struct pmem_device *pmem, struct page *page,
 			unsigned int len, unsigned int off, int rw,
 			sector_t sector)
@@ -48,11 +104,11 @@ static void pmem_do_bvec(struct pmem_device *pmem, struct page *page,
 	size_t pmem_off = sector << 9;
 
 	if (rw == READ) {
-		memcpy(mem + off, pmem->virt_addr + pmem_off, len);
+		memcpy_from_pmem(mem + off, pmem->virt_addr + pmem_off, len);
 		flush_dcache_page(page);
 	} else {
 		flush_dcache_page(page);
-		memcpy(pmem->virt_addr + pmem_off, mem + off, len);
+		pmem->ops.copy(pmem->virt_addr + pmem_off, mem + off, len);
 	}
 
 	kunmap_atomic(mem);
@@ -83,6 +139,8 @@ static void pmem_make_request(struct request_queue *q, struct bio *bio)
 		sector += bvec.bv_len >> 9;
 	}
 
+	if (rw)
+		pmem->ops.sync();
 out:
 	bio_endio(bio, err);
 }
@@ -107,7 +165,8 @@ static long pmem_direct_access(struct block_device *bdev, sector_t sector,
 	if (!pmem)
 		return -ENODEV;
 
-	*kaddr = pmem->virt_addr + offset;
+	/* FIXME convert DAX to comprehend that this mapping has a lifetime */
+	*kaddr = (void __force *) pmem->virt_addr + offset;
 	*pfn = (pmem->phys_addr + offset) >> PAGE_SHIFT;
 
 	return pmem->size - offset;
@@ -132,6 +191,8 @@ static struct pmem_device *pmem_alloc(struct device *dev, struct resource *res)
 
 	pmem->phys_addr = res->start;
 	pmem->size = resource_size(res);
+	if (!pmem_ops_init(pmem))
+		dev_warn(dev, "unable to guarantee persistence of writes\n");
 
 	err = -EINVAL;
 	if (!request_mem_region(pmem->phys_addr, pmem->size, "pmem")) {
@@ -144,7 +205,7 @@ static struct pmem_device *pmem_alloc(struct device *dev, struct resource *res)
 	 * of the CPU caches in case of a crash.
 	 */
 	err = -ENOMEM;
-	pmem->virt_addr = memremap_wt(pmem->phys_addr, pmem->size);
+	pmem->virt_addr = pmem->ops.remap(pmem->phys_addr, pmem->size);
 	if (!pmem->virt_addr)
 		goto out_release_region;
 
@@ -180,7 +241,7 @@ static struct pmem_device *pmem_alloc(struct device *dev, struct resource *res)
 out_free_queue:
 	blk_cleanup_queue(pmem->pmem_queue);
 out_unmap:
-	memunmap(pmem->virt_addr);
+	memunmap_pmem(pmem->virt_addr);
 out_release_region:
 	release_mem_region(pmem->phys_addr, pmem->size);
 out_free_dev:
@@ -194,7 +255,7 @@ static void pmem_free(struct pmem_device *pmem)
 	del_gendisk(pmem->pmem_disk);
 	put_disk(pmem->pmem_disk);
 	blk_cleanup_queue(pmem->pmem_queue);
-	memunmap(pmem->virt_addr);
+	memunmap_pmem(pmem->virt_addr);
 	release_mem_region(pmem->phys_addr, pmem->size);
 	kfree(pmem);
 }
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 05be2352fef8..26fc8bc77f85 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -21,6 +21,7 @@
 # define __rcu		__attribute__((noderef, address_space(4)))
 #else
 # define __rcu
+# define __pmem		__attribute__((noderef, address_space(5)))
 #endif
 extern void __chk_user_ptr(const volatile void __user *);
 extern void __chk_io_ptr(const volatile void __iomem *);
@@ -42,6 +43,7 @@ extern void __chk_io_ptr(const volatile void __iomem *);
 # define __cond_lock(x,c) (c)
 # define __percpu
 # define __rcu
+# define __pmem
 #endif
 
 /* Indirect macros required for expanded argument pasting, eg. __LINE__. */
diff --git a/include/linux/pmem.h b/include/linux/pmem.h
new file mode 100644
index 000000000000..0fad4ad714cc
--- /dev/null
+++ b/include/linux/pmem.h
@@ -0,0 +1,102 @@
+/*
+ * Copyright(c) 2015 Intel Corporation. All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ */
+#ifndef __PMEM_H__
+#define __PMEM_H__
+
+#include <linux/io.h>
+#include <asm/cacheflush.h>
+
+/*
+ * Architectures that define ARCH_HAS_PMEM_API must provide
+ * implementations for arch_memremap_pmem(), arch_memcpy_to_pmem(),
+ * arch_sync_pmem(), and __arch_has_sync_pmem().
+ */
+
+#ifdef CONFIG_ARCH_HAS_PMEM_API
+/**
+ * memremap_pmem - map physical persistent memory for pmem api
+ * @offset: physical address of persistent memory
+ * @size: size of the mapping
+ *
+ * Establish a mapping of the architecture specific memory type expected
+ * by memcpy_to_pmem() and sync_pmem().  For example, it may be
+ * the case that an uncacheable or writethrough mapping is sufficient,
+ * or a writeback mapping provided memcpy_to_pmem() and
+ * sync_pmem() arrange for the data to be written through the
+ * cache to persistent media.
+ */
+static inline void __pmem *memremap_pmem(resource_size_t offset, unsigned long size)
+{
+	return arch_memremap_pmem(offset, size);
+}
+
+/**
+ * memcpy_to_pmem - copy data to persistent memory
+ * @dst: destination buffer for the copy
+ * @src: source buffer for the copy
+ * @n: length of the copy in bytes
+ *
+ * Perform a memory copy that results in the destination of the copy
+ * being effectively evicted from, or never written to, the processor
+ * cache hierarchy after the copy completes.  After memcpy_to_pmem()
+ * data may still reside in cpu or platform buffers, so this operation
+ * must be followed by a sync_pmem().
+ */
+static inline void memcpy_to_pmem(void __pmem *dst, const void *src, size_t n)
+{
+	arch_memcpy_to_pmem(dst, src, n);
+}
+
+/**
+ * sync_pmem - synchronize writes to persistent memory
+ *
+ * After a series of memcpy_to_pmem() operations this drains data from
+ * cpu write buffers and any platform (memory controller) buffers to
+ * ensure that written data is durable on persistent memory media.
+ */
+static inline void sync_pmem(void)
+{
+	arch_sync_pmem();
+}
+
+/**
+ * arch_has_sync_pmem - true if sync_pmem() ensures durability
+ *
+ * For a given cpu implementation within an architecture it is possible
+ * that sync_pmem() resolves to a nop.  In the case this returns
+ * false, pmem api users are unable to ensure durabilty and may want to
+ * fall back to a different data consistency model, or otherwise notify
+ * the user.
+ */
+static inline bool arch_has_sync_pmem(void)
+{
+	return __arch_has_sync_pmem();
+}
+#else
+/* undefined symbols */
+extern void __pmem *memremap_pmem(resource_size_t offet, unsigned long size);
+extern void memcpy_to_pmem(void __pmem *dst, const void *src, size_t n);
+extern void sync_pmem(void);
+extern bool arch_has_sync_pmem(void);
+#endif /* CONFIG_ARCH_HAS_PMEM_API */
+
+static inline void memcpy_from_pmem(void *dst, void __pmem const *src, size_t size)
+{
+	memcpy(dst, (void __force const *) src, size);
+}
+
+static inline void memunmap_pmem(void __pmem *addr)
+{
+	memunmap((void __force *) addr);
+}
+#endif /* __PMEM_H__ */
diff --git a/lib/Kconfig b/lib/Kconfig
index bc7bc0278921..0d28cc560c6b 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -525,4 +525,7 @@ config ARCH_HAS_SG_CHAIN
 config ARCH_HAS_MEMREMAP
 	bool
 
+config ARCH_HAS_PMEM_API
+	bool
+
 endmenu

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