Re: [PATCH 8/9] cxl/core: Add dax_kmem_region and sysram_region drivers
From: Jonathan Cameron <jonathan.cameron@huawei.com>
Date: 2026-02-02 18:20:20
Also in:
linux-cxl, linux-fsdevel, linux-mm, lkml, nvdimm
On Thu, 29 Jan 2026 16:04:41 -0500 Gregory Price [off-list ref] wrote:
In the current kmem driver binding process, the only way for users
to define hotplug policy is via a build-time option, or by not
onlining memory by default and setting each individual memory block
online after hotplug occurs. We can solve this with a configuration
step between region-probe and dax-probe.
Add the infrastructure for a two-stage driver binding for kmem-mode
dax regions. The cxl_dax_kmem_region driver probes cxl_sysram_region
devices and creates cxl_dax_region with dax_driver=kmem.
This creates an interposition step where users can configure policy.
Device hierarchy:
region0 -> sysram_region0 -> dax_region0 -> dax0.0
The sysram_region device exposes a sysfs 'online_type' attribute
that allows users to configure the memory online type before the
underlying dax_region is created and memory is hotplugged.
sysram_region0/online_type:
invalid: not configured, blocks probe
offline: memory will not be onlined automatically
online: memory will be onlined in ZONE_NORMAL
online_movable: memory will be onlined in ZONE_MMOVABLEZONE_MOVABLE
The device initializes with online_type=invalid which prevents the cxl_dax_kmem_region driver from binding until the user explicitly configures a valid online_type. This enables a two-step binding process: echo region0 > cxl_sysram_region/bind echo online_movable > sysram_region0/online_type echo sysram_region0 > cxl_dax_kmem_region/bind Signed-off-by: Gregory Price <gourry@gourry.net>
Trivial stuff. Will mull over this series as a whole... My first instinctive reaction is positive - I'm just wondering where additional drivers fit into this and whether it has the right degree of flexibility.
quoted hunk ↗ jump to hunk
diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c index 6200ca1cc2dd..8bef91dc726c 100644 --- a/drivers/cxl/core/region.c +++ b/drivers/cxl/core/region.c@@ -3734,8 +3734,20 @@ int cxl_region_init(void) if (rc) goto err_dax; + rc = cxl_driver_register(&cxl_sysram_region_driver);
This smells like a loop over an array of drivers is becoming sensible.
quoted hunk ↗ jump to hunk
+ if (rc) + goto err_sysram; + + rc = cxl_driver_register(&cxl_dax_kmem_region_driver); + if (rc) + goto err_dax_kmem; + return 0; +err_dax_kmem: + cxl_driver_unregister(&cxl_sysram_region_driver); +err_sysram: + cxl_driver_unregister(&cxl_devdax_region_driver); err_dax: cxl_driver_unregister(&cxl_region_driver); return rc;@@ -3743,6 +3755,8 @@ int cxl_region_init(void) void cxl_region_exit(void) { + cxl_driver_unregister(&cxl_dax_kmem_region_driver); + cxl_driver_unregister(&cxl_sysram_region_driver); cxl_driver_unregister(&cxl_devdax_region_driver); cxl_driver_unregister(&cxl_region_driver); }diff --git a/drivers/cxl/core/sysram_region.c b/drivers/cxl/core/sysram_region.c new file mode 100644 index 000000000000..5665db238d0f --- /dev/null +++ b/drivers/cxl/core/sysram_region.c@@ -0,0 +1,180 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* Copyright(c) 2026 Meta Platforms, Inc. All rights reserved. */ +/* + * CXL Sysram Region - Intermediate device for kmem hotplug configuration + * + * This provides an intermediate device between cxl_region and cxl_dax_region + * that allows users to configure memory hotplug parameters (like online_type) + * before the underlying dax_region is created and memory is hotplugged. + */ + +#include <linux/memory_hotplug.h> +#include <linux/device.h> +#include <linux/slab.h> +#include <cxlmem.h> +#include <cxl.h> +#include "core.h"
+
+static DEVICE_ATTR_RW(online_type);
+
+static struct attribute *cxl_sysram_region_attrs[] = {
+ &dev_attr_online_type.attr,
+ NULL,As below.
+};
+
+static const struct attribute_group cxl_sysram_region_attribute_group = {
+ .attrs = cxl_sysram_region_attrs,
+};
+
+static const struct attribute_group *cxl_sysram_region_attribute_groups[] = {
+ &cxl_base_attribute_group,
+ &cxl_sysram_region_attribute_group,
+ NULL,Trivial, but don't want a comma on that NULL.
+};
quoted hunk ↗ jump to hunk
diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h index 674d5f870c70..1544c27e9c89 100644 --- a/drivers/cxl/cxl.h +++ b/drivers/cxl/cxl.h@@ -596,6 +596,25 @@ struct cxl_dax_region { enum dax_driver_type dax_driver; }; +/** + * struct cxl_sysram_region - CXL RAM region for system memory hotplug + * @dev: device for this sysram_region + * @cxlr: parent cxl_region + * @hpa_range: Host physical address range for the region + * @online_type: Memory online type (MMOP_* 0-3, or -1 if not configured)
Ah. An there's our reason for an int. Can we just add a MMOP enum value for not configured yet and so let us use it as an enum? Or have a separate bool for that and ignore the online_type until it's set.
+ *
+ * Intermediate device that allows configuration of memory hotplug
+ * parameters before the underlying dax_region is created. The device
+ * starts with online_type=-1 which prevents the cxl_dax_kmem_region
+ * driver from binding until the user explicitly sets online_type.
+ */
+struct cxl_sysram_region {
+ struct device dev;
+ struct cxl_region *cxlr;
+ struct range hpa_range;
+ int online_type;
+};