cxl_test: Limit location for fake CFMWS to mappable range

Some architectures (e.g. arm64) only support memory hotplug operations on
a restricted set of physical addresses. This applies even when we are
faking some CXL fixed memory windows for the purposes of cxl_test.
That range can be queried with mhp_get_pluggable_range(true). Use the
minimum of that the top of that range and iomem_resource.end to establish
the 64GiB region used by cxl_test.

From thread #2 which was related to the issue in #1.

[ dj: Add CONFIG_MEMORY_HOTPLUG config check, from Alison ]

Link: https://lore.kernel.org/linux-cxl/20250522145622.00002633@huawei.com/ #2
Reported-by: Itaru Kitayama <itaru.kitayama@linux.dev>
Closes: pmem/ndctl#278 #1
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Itaru Kitayama <itaru.kitayama@fujitsu.com <mailto:itaru.kitayama@fujitsu.com>
Tested-by: Marc Herbert <marc.herbert@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://patch.msgid.link/20250527153451.82858-1-Jonathan.Cameron@huawei.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: jic23

tools/testing/cxl/config_check.c

@@ -14,4 +14,5 @@ void check(void)
 	BUILD_BUG_ON(!IS_ENABLED(CONFIG_CXL_REGION_INVALIDATION_TEST));
 	BUILD_BUG_ON(!IS_ENABLED(CONFIG_NVDIMM_SECURITY_TEST));
 	BUILD_BUG_ON(!IS_ENABLED(CONFIG_DEBUG_FS));
+	BUILD_BUG_ON(!IS_ENABLED(CONFIG_MEMORY_HOTPLUG));
 }

tools/testing/cxl/test/cxl.c

@@ -2,6 +2,7 @@
 // Copyright(c) 2021 Intel Corporation. All rights reserved.
 
 #include <linux/platform_device.h>
+#include <linux/memory_hotplug.h>
 #include <linux/genalloc.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
@@ -1328,6 +1329,7 @@ static int cxl_mem_init(void)
 static __init int cxl_test_init(void)
 {
 	int rc, i;
+	struct range mappable;
 
 	cxl_acpi_test();
 	cxl_core_test();
@@ -1342,8 +1344,11 @@ static __init int cxl_test_init(void)
 		rc = -ENOMEM;
 		goto err_gen_pool_create;
 	}
+	mappable = mhp_get_pluggable_range(true);
 
-	rc = gen_pool_add(cxl_mock_pool, iomem_resource.end + 1 - SZ_64G,
+	rc = gen_pool_add(cxl_mock_pool,
+			  min(iomem_resource.end + 1 - SZ_64G,
+			      mappable.end + 1 - SZ_64G),
 			  SZ_64G, NUMA_NO_NODE);
 	if (rc)
 		goto err_gen_pool_add;