[PATCH 2/4] mm: add a io_mapping_map_user helper

[Christoph Hellwig <hch@lst.de>]

Add a helper that calls remap_pfn_range for an struct io_mapping, relying
on the pgprot pre-validation done when creating the mapping instead of
doing it at runtime.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 include/linux/io-mapping.h |  3 +++
 mm/Kconfig                 |  3 +++
 mm/Makefile                |  1 +
 mm/io-mapping.c            | 29 +++++++++++++++++++++++++++++
 4 files changed, 36 insertions(+)
 create mode 100644 mm/io-mapping.c

diff --git a/include/linux/io-mapping.h b/include/linux/io-mapping.h
index c093e81310a9b3..e9743cfd858527 100644
--- a/include/linux/io-mapping.h
+++ b/include/linux/io-mapping.h
@@ -220,3 +220,6 @@ io_mapping_free(struct io_mapping *iomap)
 }
 
 #endif /* _LINUX_IO_MAPPING_H */
+
+int io_mapping_map_user(struct io_mapping *iomap, struct vm_area_struct *vma,
+		unsigned long addr, unsigned long pfn, unsigned long size);
diff --git a/mm/Kconfig b/mm/Kconfig
index 24c045b24b9506..6b0f2cfbfac0f3 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -872,4 +872,7 @@ config MAPPING_DIRTY_HELPERS
 config KMAP_LOCAL
 	bool
 
+# struct io_mapping based helper.  Selected by drivers that need them
+config IO_MAPPING
+	bool
 endmenu
diff --git a/mm/Makefile b/mm/Makefile
index 72227b24a61688..c0135e385984bb 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -120,3 +120,4 @@ obj-$(CONFIG_MEMFD_CREATE) += memfd.o
 obj-$(CONFIG_MAPPING_DIRTY_HELPERS) += mapping_dirty_helpers.o
 obj-$(CONFIG_PTDUMP_CORE) += ptdump.o
 obj-$(CONFIG_PAGE_REPORTING) += page_reporting.o
+obj-$(CONFIG_IO_MAPPING) += io-mapping.o
diff --git a/mm/io-mapping.c b/mm/io-mapping.c
new file mode 100644
index 00000000000000..01b3627999304e
--- /dev/null
+++ b/mm/io-mapping.c
@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/mm.h>
+#include <linux/io-mapping.h>
+
+/**
+ * io_mapping_map_user - remap an I/O mapping to userspace
+ * @iomap: the source io_mapping
+ * @vma: user vma to map to
+ * @addr: target user address to start at
+ * @pfn: physical address of kernel memory
+ * @size: size of map area
+ *
+ *  Note: this is only safe if the mm semaphore is held when called.
+ */
+int io_mapping_map_user(struct io_mapping *iomap, struct vm_area_struct *vma,
+		unsigned long addr, unsigned long pfn, unsigned long size)
+{
+	vm_flags_t expected_flags = VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
+
+	if (WARN_ON_ONCE((vma->vm_flags & expected_flags) != expected_flags))
+		return -EINVAL;
+
+	/* We rely on prevalidation of the io-mapping to skip track_pfn(). */
+	return remap_pfn_range_notrack(vma, addr, pfn, size,
+		__pgprot((pgprot_val(iomap->prot) & _PAGE_CACHE_MASK) |
+			 (pgprot_val(vma->vm_page_prot) & ~_PAGE_CACHE_MASK)));
+}
+EXPORT_SYMBOL_GPL(io_mapping_map_user);
-- 
2.30.1