numa: Add simple generic NUMA emulation

Add some common code for splitting the memory into N emulated NUMA memory
nodes.

Individual architecture can then enable selecting this option and use the
existing numa=fake=<N> kernel argument to enable it.

Memory is always split into equally sized chunks.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Co-developed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: “Rafael J. Wysocki" <rafael@kernel.org>

drivers/base/Kconfig

@@ -230,6 +230,13 @@ config GENERIC_ARCH_NUMA
 	  Enable support for generic NUMA implementation. Currently, RISC-V
 	  and ARM64 use it.
 
+config GENERIC_ARCH_NUMA_EMULATION
+	bool
+	depends on GENERIC_ARCH_NUMA
+	help
+	  Enable NUMA emulation. Note that NUMA emulation will only be used if
+	  the machine has no NUMA node.
+
 config FW_DEVLINK_SYNC_STATE_TIMEOUT
 	bool "sync_state() behavior defaults to timeout instead of strict"
 	help

drivers/base/Makefile

@@ -25,6 +25,7 @@ obj-$(CONFIG_DEV_COREDUMP) += devcoredump.o
 obj-$(CONFIG_GENERIC_MSI_IRQ) += platform-msi.o
 obj-$(CONFIG_GENERIC_ARCH_TOPOLOGY) += arch_topology.o
 obj-$(CONFIG_GENERIC_ARCH_NUMA) += arch_numa.o
+obj-$(CONFIG_GENERIC_ARCH_NUMA_EMULATION) += numa_emulation.o
 obj-$(CONFIG_ACPI) += physical_location.o
 
 obj-y			+= test/

drivers/base/arch_numa.c

@@ -15,6 +15,8 @@
 
 #include <asm/sections.h>
 
+#include "numa_emulation.h"
+
 struct pglist_data *node_data[MAX_NUMNODES] __read_mostly;
 EXPORT_SYMBOL(node_data);
 nodemask_t numa_nodes_parsed __initdata;
@@ -30,6 +32,8 @@ static __init int numa_parse_early_param(char *opt)
 		return -EINVAL;
 	if (str_has_prefix(opt, "off"))
 		numa_off = true;
+	if (str_has_prefix(opt, "fake="))
+		return numa_emu_cmdline(opt + 5);
 
 	return 0;
 }
@@ -471,6 +475,8 @@ void __init arch_numa_init(void)
 			return;
 		if (acpi_disabled && !numa_init(of_numa_init))
 			return;
+		if (!numa_init(numa_emu_init))
+			return;
 	}
 
 	numa_init(dummy_numa_init);

drivers/base/numa_emulation.c

@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Simple NUMA emulation.
+ *
+ * Copyright © 2024 Raspberry Pi Ltd
+ *
+ * Author: Maíra Canal <mcanal@igalia.com>
+ * Author: Tvrtko Ursulin <tursulin@igalia.com>
+ */
+#include <linux/memblock.h>
+
+#include "numa_emulation.h"
+
+static unsigned int emu_nodes;
+
+int __init numa_emu_cmdline(char *str)
+{
+	int ret;
+
+	ret = kstrtouint(str, 10, &emu_nodes);
+	if (ret)
+		return ret;
+
+	if (emu_nodes > MAX_NUMNODES) {
+		pr_notice("numa=fake=%u too large, reducing to %u\n",
+			  emu_nodes, MAX_NUMNODES);
+		emu_nodes = MAX_NUMNODES;
+	}
+
+	return 0;
+}
+
+int __init numa_emu_init(void)
+{
+	phys_addr_t start, end;
+	unsigned long size;
+	unsigned int i;
+	int ret;
+
+	if (!emu_nodes)
+		return -EINVAL;
+
+	start = memblock_start_of_DRAM();
+	end = memblock_end_of_DRAM() - 1;
+
+	size = DIV_ROUND_DOWN_ULL(end - start + 1, emu_nodes);
+	size = PAGE_ALIGN_DOWN(size);
+
+	for (i = 0; i < emu_nodes; i++) {
+		u64 s, e;
+
+		s = start + i * size;
+		e = s + size - 1;
+
+		if (i == (emu_nodes - 1) && e != end)
+			e = end;
+
+		pr_info("Faking a node at [mem %pap-%pap]\n", &s, &e);
+		ret = numa_add_memblk(i, s, e + 1);
+		if (ret) {
+			pr_err("Failed to add fake NUMA node %d!\n", i);
+			break;
+		}
+	}
+
+	return ret;
+}

drivers/base/numa_emulation.h

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * NUMA emulation header
+ *
+ * Copyright © 2024 Raspberry Pi Ltd
+ */
+
+#ifdef CONFIG_GENERIC_ARCH_NUMA_EMULATION
+int numa_emu_cmdline(char *str);
+int __init numa_emu_init(void);
+#else
+static inline int numa_emu_cmdline(char *str)
+{
+	return -EINVAL;
+}
+
+static int __init numa_emu_init(void)
+{
+	return -EOPNOTSUPP;
+}
+#endif /* CONFIG_NUMA_EMU */