linux-kernel - [RFC PATCH 12/17] cxl: docs/allocation/dax

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Message-ID: <20250430001224.1028656-13-gourry@gourry.net>
Date: Tue, 29 Apr 2025 20:12:19 -0400
From: Gregory Price <gourry@...rry.net>
To: linux-cxl@...r.kernel.org
Cc: linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	dave@...olabs.net,
	jonathan.cameron@...wei.com,
	dave.jiang@...el.com,
	alison.schofield@...el.com,
	vishal.l.verma@...el.com,
	ira.weiny@...el.com,
	dan.j.williams@...el.com,
	corbet@....net
Subject: [RFC PATCH 12/17] cxl: docs/allocation/dax

Small example of accessing CXL memory capacity via DAX device

Signed-off-by: Gregory Price <gourry@...rry.net>
---
 .../driver-api/cxl/allocation/dax.rst         | 59 +++++++++++++++++++
 Documentation/driver-api/cxl/index.rst        |  5 ++
 2 files changed, 64 insertions(+)
 create mode 100644 Documentation/driver-api/cxl/allocation/dax.rst

diff --git a/Documentation/driver-api/cxl/allocation/dax.rst b/Documentation/driver-api/cxl/allocation/dax.rst
new file mode 100644
index 000000000000..8e0c9f6a6843
--- /dev/null
+++ b/Documentation/driver-api/cxl/allocation/dax.rst
@@ -0,0 +1,59 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+DAX Devices
+###########
+CXL capacity exposed as a DAX device can be accessed directly via mmap.
+Users may wish to use this interface mechanism to write their own userland
+CXL allocator, or to managed shared or persistent memory regions across multiple
+hosts.
+
+If the capacity is shared across hosts or persistent, appropriate flushing
+mechanisms must be employed unless the region supports Snoop Back-Invalidate.
+
+Note that mappings must be aligned (size and base) to the dax device's base
+alignment, which is typically 2MB - but maybe be configured larger.
+
+::
+
+  #include <stdio.h>
+  #include <stdlib.h>
+  #include <stdint.h>
+  #include <sys/mman.h>
+  #include <fcntl.h>
+  #include <unistd.h>
+
+  #define DEVICE_PATH "/dev/dax0.0" // Replace DAX device path
+  #define DEVICE_SIZE (4ULL * 1024 * 1024 * 1024) // 4GB
+
+  int main() {
+      int fd;
+      void* mapped_addr;
+
+      /* Open the DAX device */
+      fd = open(DEVICE_PATH, O_RDWR);
+      if (fd < 0) {
+          perror("open");
+          return -1;
+      }
+
+      /* Map the device into memory */
+      mapped_addr = mmap(NULL, DEVICE_SIZE, PROT_READ | PROT_WRITE,
+                         MAP_SHARED, fd, 0);
+      if (mapped_addr == MAP_FAILED) {
+          perror("mmap");
+          close(fd);
+          return -1;
+      }
+
+      printf("Mapped address: %p\n", mapped_addr);
+
+      /* You can now access the device through the mapped address */
+      uint64_t* ptr = (uint64_t*)mapped_addr;
+      *ptr = 0x1234567890abcdef; // Write a value to the device
+      printf("Value at address %p: 0x%016llx\n", ptr, *ptr);
+
+      /* Clean up */
+      munmap(mapped_addr, DEVICE_SIZE);
+      close(fd);
+      return 0;
+  }
diff --git a/Documentation/driver-api/cxl/index.rst b/Documentation/driver-api/cxl/index.rst
index d5186fc609a9..d19148be3087 100644
--- a/Documentation/driver-api/cxl/index.rst
+++ b/Documentation/driver-api/cxl/index.rst
@@ -42,5 +42,10 @@ that have impacts on each other.  The docs here break up configurations steps.
    linux/memory-hotplug
    linux/access-coordinates
 
+.. toctree::
+   :maxdepth: 2
+   :caption: Memory Allocation
+
+   allocation/dax
 
 .. only::  subproject and html
-- 
2.49.0