[PATCH v2 3/4] cxl, acpi/hmat: Update CXL access coordinates directly instead of through HMAT

[PATCH v2 3/4] cxl, acpi/hmat: Update CXL access coordinates directly instead of through HMAT

[Dave Jiang]

The current implementation of CXL memory hotplug notifier gets called
before the HMAT memory hotplug notifier. The CXL driver calculates the
access coordinates (bandwidth and latency values) for the CXL end to
end path (i.e. CPU to endpoint). When the CXL region is onlined, the CXL
memory hotplug notifier writes the access coordinates to the HMAT target
structs. Then the HMAT memory hotplug notifier is called and it creates
the access coordinates for the node sysfs attributes.

During testing on an Intel platform, it was found that although the
newly calculated coordinates were pushed to sysfs, the sysfs attributes for
the access coordinates showed up with the wrong initiator. The system has
4 nodes (0, 1, 2, 3) where node 0 and 1 are CPU nodes and node 2 and 3 are
CXL nodes. The expectation is that node 2 would show up as a target to node
0:
/sys/devices/system/node/node2/access0/initiators/node0

However it was observed that node 2 showed up as a target under node 1:
/sys/devices/system/node/node2/access0/initiators/node1

The original intent of the 'ext_updated' flag in HMAT handling code was to
stop HMAT memory hotplug callback from clobbering the access coordinates
after CXL has injected its calculated coordinates and replaced the generic
target access coordinates provided by the HMAT table in the HMAT target
structs. However the flag is hacky at best and blocks the updates from
other CXL regions that are onlined in the same node later on. Remove the
'ext_updated' flag usage and just update the access coordinates for the
nodes directly without touching HMAT target data.

The hotplug memory callback ordering is changed. Instead of changing CXL,
move HMAT back so there's room for the levels rather than have CXL share
the same level as SLAB_CALLBACK_PRI. The change will resulting in the CXL
callback to be executed after the HMAT callback.

With the change, the CXL hotplug memory notifier runs after the HMAT
callback. The HMAT callback will create the node sysfs attributes for
access coordinates. The CXL callback will write the access coordinates to
the now created node sysfs attributes directly and will not pollute the
HMAT target values.

Fixes: 067353a46d8c ("cxl/region: Add memory hotplug notifier for cxl region")
Cc: stable@vger.kernel.org
Tested-by: Marc Herbert <marc.herbert@linux.intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
---
v2:
- Add description to what was observed for the issue. (Dan)
- Use the correct Fixes tag. (Dan)
- Add Cc to stable. (Dan)
- Add support to only update on first region appearance. (Jonathan)
---
 drivers/acpi/numa/hmat.c  |  6 ------
 drivers/cxl/core/cdat.c   |  5 -----
 drivers/cxl/core/core.h   |  1 -
 drivers/cxl/core/region.c | 21 +++++++++++++--------
 include/linux/memory.h    |  2 +-
 5 files changed, 14 insertions(+), 21 deletions(-)

diff --git a/drivers/acpi/numa/hmat.c b/drivers/acpi/numa/hmat.c
index 4958301f5417..5d32490dc4ab 100644
--- a/drivers/acpi/numa/hmat.c
+++ b/drivers/acpi/numa/hmat.c
@@ -74,7 +74,6 @@ struct memory_target {
 	struct node_cache_attrs cache_attrs;
 	u8 gen_port_device_handle[ACPI_SRAT_DEVICE_HANDLE_SIZE];
 	bool registered;
-	bool ext_updated;	/* externally updated */
 };
 
 struct memory_initiator {
@@ -391,7 +390,6 @@ int hmat_update_target_coordinates(int nid, struct access_coordinate *coord,
 				  coord->read_bandwidth, access);
 	hmat_update_target_access(target, ACPI_HMAT_WRITE_BANDWIDTH,
 				  coord->write_bandwidth, access);
-	target->ext_updated = true;
 
 	return 0;
 }
@@ -773,10 +771,6 @@ static void hmat_update_target_attrs(struct memory_target *target,
 	u32 best = 0;
 	int i;
 
-	/* Don't update if an external agent has changed the data.  */
-	if (target->ext_updated)
-		return;
-
 	/* Don't update for generic port if there's no device handle */
 	if ((access == NODE_ACCESS_CLASS_GENPORT_SINK_LOCAL ||
 	     access == NODE_ACCESS_CLASS_GENPORT_SINK_CPU) &&
diff --git a/drivers/cxl/core/cdat.c b/drivers/cxl/core/cdat.c
index c0af645425f4..c891fd618cfd 100644
--- a/drivers/cxl/core/cdat.c
+++ b/drivers/cxl/core/cdat.c
@@ -1081,8 +1081,3 @@ int cxl_update_hmat_access_coordinates(int nid, struct cxl_region *cxlr,
 {
 	return hmat_update_target_coordinates(nid, &cxlr->coord[access], access);
 }
-
-bool cxl_need_node_perf_attrs_update(int nid)
-{
-	return !acpi_node_backed_by_real_pxm(nid);
-}
diff --git a/drivers/cxl/core/core.h b/drivers/cxl/core/core.h
index 2669f251d677..a253d308f3c9 100644
--- a/drivers/cxl/core/core.h
+++ b/drivers/cxl/core/core.h
@@ -139,7 +139,6 @@ long cxl_pci_get_latency(struct pci_dev *pdev);
 int cxl_pci_get_bandwidth(struct pci_dev *pdev, struct access_coordinate *c);
 int cxl_update_hmat_access_coordinates(int nid, struct cxl_region *cxlr,
 				       enum access_coordinate_class access);
-bool cxl_need_node_perf_attrs_update(int nid);
 int cxl_port_get_switch_dport_bandwidth(struct cxl_port *port,
 					struct access_coordinate *c);
 
diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 71cc42d05248..371873fc43eb 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -9,6 +9,7 @@
 #include <linux/uuid.h>
 #include <linux/sort.h>
 #include <linux/idr.h>
+#include <linux/xarray.h>
 #include <linux/memory-tiers.h>
 #include <cxlmem.h>
 #include <cxl.h>
@@ -30,6 +31,9 @@
  * 3. Decoder targets
  */
 
+/* xarray that stores the reference count per node for regions */
+static DEFINE_XARRAY(node_regions_xa);
+
 static struct cxl_region *to_cxl_region(struct device *dev);
 
 #define __ACCESS_ATTR_RO(_level, _name) {				\
@@ -2442,14 +2446,8 @@ static bool cxl_region_update_coordinates(struct cxl_region *cxlr, int nid)
 
 	for (int i = 0; i < ACCESS_COORDINATE_MAX; i++) {
 		if (cxlr->coord[i].read_bandwidth) {
-			rc = 0;
-			if (cxl_need_node_perf_attrs_update(nid))
-				node_set_perf_attrs(nid, &cxlr->coord[i], i);
-			else
-				rc = cxl_update_hmat_access_coordinates(nid, cxlr, i);
-
-			if (rc == 0)
-				cset++;
+			node_update_perf_attrs(nid, &cxlr->coord[i], i);
+			cset++;
 		}
 	}
 
@@ -2475,6 +2473,7 @@ static int cxl_region_perf_attrs_callback(struct notifier_block *nb,
 	struct node_notify *nn = arg;
 	int nid = nn->nid;
 	int region_nid;
+	int rc;
 
 	if (action != NODE_ADDED_FIRST_MEMORY)
 		return NOTIFY_DONE;
@@ -2487,6 +2486,11 @@ static int cxl_region_perf_attrs_callback(struct notifier_block *nb,
 	if (nid != region_nid)
 		return NOTIFY_DONE;
 
+	/* No action needed if there's existing entry */
+	rc = xa_insert(&node_regions_xa, nid, NULL, GFP_KERNEL);
+	if (rc < 0)
+		return NOTIFY_DONE;
+
 	if (!cxl_region_update_coordinates(cxlr, nid))
 		return NOTIFY_DONE;
 
@@ -3638,6 +3642,7 @@ int cxl_region_init(void)
 
 void cxl_region_exit(void)
 {
+	xa_destroy(&node_regions_xa);
 	cxl_driver_unregister(&cxl_region_driver);
 }
 
diff --git a/include/linux/memory.h b/include/linux/memory.h
index de5c0d8e8925..684fd641f2f0 100644
--- a/include/linux/memory.h
+++ b/include/linux/memory.h
@@ -120,8 +120,8 @@ struct mem_section;
  */
 #define DEFAULT_CALLBACK_PRI	0
 #define SLAB_CALLBACK_PRI	1
-#define HMAT_CALLBACK_PRI	2
 #define CXL_CALLBACK_PRI	5
+#define HMAT_CALLBACK_PRI	6
 #define MM_COMPUTE_BATCH_PRI	10
 #define CPUSET_CALLBACK_PRI	10
 #define MEMTIER_HOTPLUG_PRI	100
-- 
2.50.1

Re: [PATCH v2 3/4] cxl, acpi/hmat: Update CXL access coordinates directly instead of through HMAT

[Jonathan Cameron]

On Wed, 20 Aug 2025 12:47:03 -0700
Dave Jiang <dave.jiang@intel.com> wrote:

> The current implementation of CXL memory hotplug notifier gets called
> before the HMAT memory hotplug notifier. The CXL driver calculates the
> access coordinates (bandwidth and latency values) for the CXL end to
> end path (i.e. CPU to endpoint). When the CXL region is onlined, the CXL
> memory hotplug notifier writes the access coordinates to the HMAT target
> structs. Then the HMAT memory hotplug notifier is called and it creates
> the access coordinates for the node sysfs attributes.
> 
> During testing on an Intel platform, it was found that although the
> newly calculated coordinates were pushed to sysfs, the sysfs attributes for
> the access coordinates showed up with the wrong initiator. The system has
> 4 nodes (0, 1, 2, 3) where node 0 and 1 are CPU nodes and node 2 and 3 are
> CXL nodes. The expectation is that node 2 would show up as a target to node
> 0:
> /sys/devices/system/node/node2/access0/initiators/node0
> 
> However it was observed that node 2 showed up as a target under node 1:
> /sys/devices/system/node/node2/access0/initiators/node1
> 
> The original intent of the 'ext_updated' flag in HMAT handling code was to
> stop HMAT memory hotplug callback from clobbering the access coordinates
> after CXL has injected its calculated coordinates and replaced the generic
> target access coordinates provided by the HMAT table in the HMAT target
> structs. However the flag is hacky at best and blocks the updates from
> other CXL regions that are onlined in the same node later on. Remove the
> 'ext_updated' flag usage and just update the access coordinates for the
> nodes directly without touching HMAT target data.
> 
> The hotplug memory callback ordering is changed. Instead of changing CXL,
> move HMAT back so there's room for the levels rather than have CXL share
> the same level as SLAB_CALLBACK_PRI. The change will resulting in the CXL
> callback to be executed after the HMAT callback.
> 
> With the change, the CXL hotplug memory notifier runs after the HMAT
> callback. The HMAT callback will create the node sysfs attributes for
> access coordinates. The CXL callback will write the access coordinates to
> the now created node sysfs attributes directly and will not pollute the
> HMAT target values.
> 
> Fixes: 067353a46d8c ("cxl/region: Add memory hotplug notifier for cxl region")
> Cc: stable@vger.kernel.org
> Tested-by: Marc Herbert <marc.herbert@linux.intel.com>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---
> v2:
> - Add description to what was observed for the issue. (Dan)
> - Use the correct Fixes tag. (Dan)
> - Add Cc to stable. (Dan)
> - Add support to only update on first region appearance. (Jonathan)

The implementation of this seems like overkill. See later, but in short
should be able to use a nodemask_t

> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 71cc42d05248..371873fc43eb 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -9,6 +9,7 @@
>  #include <linux/uuid.h>
>  #include <linux/sort.h>
>  #include <linux/idr.h>
> +#include <linux/xarray.h>
>  #include <linux/memory-tiers.h>
>  #include <cxlmem.h>
>  #include <cxl.h>
> @@ -30,6 +31,9 @@
>   * 3. Decoder targets
>   */
>  
> +/* xarray that stores the reference count per node for regions */

Reference count seems an over the top description.
I think it's just a flag that says if this happened already or not.
The term reference count would kind of suggest it would count regions
present.

So perhaps a comment along the lines of

/* xarray that stores if a region has previously been seen in a node */
and rename to node_region_seen_xa.

However, can we just use a bitmap sized to MAX_NUMNODES which is 
a nodemask_t which gives us the helpers nodemask.h



> +static DEFINE_XARRAY(node_regions_xa);
> +
>  static struct cxl_region *to_cxl_region(struct device *dev);
>  
>  #define __ACCESS_ATTR_RO(_level, _name) {				\
> @@ -2442,14 +2446,8 @@ static bool cxl_region_update_coordinates(struct cxl_region *cxlr, int nid)
>  
>  	for (int i = 0; i < ACCESS_COORDINATE_MAX; i++) {
>  		if (cxlr->coord[i].read_bandwidth) {
> -			rc = 0;
> -			if (cxl_need_node_perf_attrs_update(nid))
> -				node_set_perf_attrs(nid, &cxlr->coord[i], i);
> -			else
> -				rc = cxl_update_hmat_access_coordinates(nid, cxlr, i);
> -
> -			if (rc == 0)
> -				cset++;
> +			node_update_perf_attrs(nid, &cxlr->coord[i], i);
> +			cset++;
>  		}
>  	}
>  
> @@ -2475,6 +2473,7 @@ static int cxl_region_perf_attrs_callback(struct notifier_block *nb,
>  	struct node_notify *nn = arg;
>  	int nid = nn->nid;
>  	int region_nid;
> +	int rc;
>  
>  	if (action != NODE_ADDED_FIRST_MEMORY)
>  		return NOTIFY_DONE;
> @@ -2487,6 +2486,11 @@ static int cxl_region_perf_attrs_callback(struct notifier_block *nb,
>  	if (nid != region_nid)
>  		return NOTIFY_DONE;
>  
> +	/* No action needed if there's existing entry */
> +	rc = xa_insert(&node_regions_xa, nid, NULL, GFP_KERNEL);

So this is using the NULL entry quirk of xa_insert() where
a reserved entry is put in place so next time we match. 

> +	if (rc < 0)
> +		return NOTIFY_DONE;
> +
>  	if (!cxl_region_update_coordinates(cxlr, nid))
>  		return NOTIFY_DONE;
>  
> @@ -3638,6 +3642,7 @@ int cxl_region_init(void)
>  
>  void cxl_region_exit(void)
>  {
> +	xa_destroy(&node_regions_xa);
>  	cxl_driver_unregister(&cxl_region_driver);
>  }