riscv: add multikernel architecture skeleton

arch/riscv/Kbuild

@@ -7,5 +7,7 @@ obj-$(CONFIG_KVM) += kvm/
 
 obj-$(CONFIG_ARCH_SUPPORTS_KEXEC_PURGATORY) += purgatory/
 
+obj-$(CONFIG_MULTIKERNEL) += multikernel/
+
 # for cleaning
 subdir- += boot

arch/riscv/Kconfig

@@ -1087,6 +1087,14 @@ config ARCH_SELECTS_KEXEC_FILE
 config ARCH_SUPPORTS_KEXEC_PURGATORY
 	def_bool ARCH_SUPPORTS_KEXEC_FILE
 
+config ARCH_SUPPORTS_MULTIKERNEL
+	def_bool y
+	depends on 64BIT
+	depends on RISCV_SBI
+	depends on KEXEC_FILE
+	depends on HOTPLUG_CPU
+	depends on !RISCV_M_MODE
+
 config ARCH_SUPPORTS_CRASH_DUMP
 	def_bool y
 

arch/riscv/include/asm/multikernel.h

@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _ASM_RISCV_MULTIKERNEL_H
+#define _ASM_RISCV_MULTIKERNEL_H
+
+#ifndef __ASSEMBLY__
+
+#include <linux/errno.h>
+#include <linux/sizes.h>
+#include <linux/types.h>
+
+#include <asm/page.h>
+#include <asm/smp.h>
+
+/*
+ * Hart IDs are sparse firmware identifiers and may exceed NR_CPUS. Keep
+ * them as values and always translate through the architecture CPU maps.
+ */
+static inline u64 arch_cpu_physical_id(int cpu)
+{
+	return cpuid_to_hartid_map(cpu);
+}
+
+static inline int arch_cpu_from_physical_id(u64 hartid)
+{
+	if (hartid == INVALID_HARTID)
+		return -ENOENT;
+
+	return riscv_hartid_to_cpuid(hartid);
+}
+
+/*
+ * The RISC-V spawn path will use one page for its context, up to 64 KiB
+ * for the generated DTB, and one page for the fence.i entry stub. SBI HSM
+ * starts a hart in the existing address space, so no trampoline page tables
+ * are needed.
+ */
+#define MK_CTRL_BLOCK_SIZE	(SZ_64K + 2 * PAGE_SIZE)
+
+/*
+ * Architecture-private spawn state is added with the SBI HSM and Image
+ * loader support. The compile-only skeleton intentionally has none.
+ */
+struct mk_instance_arch {
+};
+
+#endif /* !__ASSEMBLY__ */
+
+#endif /* _ASM_RISCV_MULTIKERNEL_H */

arch/riscv/multikernel/Makefile

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+obj-y += spawn.o

arch/riscv/multikernel/spawn.c

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * RISC-V multikernel architecture interface skeleton.
+ *
+ * The SBI HSM spawn and park implementation is added by the follow-up
+ * architecture patches. Until then, operations which would change CPU
+ * ownership fail explicitly instead of pretending that a hart moved.
+ */
+
+#include <linux/errno.h>
+#include <linux/kernel.h>
+#include <linux/kexec.h>
+#include <linux/multikernel.h>
+
+void mk_arch_send_ipi(mk_phys_cpu_t phys_cpu)
+{
+	pr_warn_once("RISC-V multikernel IPI support is not implemented\n");
+}
+
+void mk_arch_register_cpu(mk_phys_cpu_t phys_id)
+{
+	/* RISC-V CPU topology already records possible harts. */
+}
+
+void __noreturn mk_enter_pool_state(void *info)
+{
+	/* Unreachable while CONFIG_ARCH_HAS_MK_POOL_STATE is disabled. */
+	panic("RISC-V multikernel pool parking is not implemented");
+}
+
+int mk_arch_register_force_stop(void)
+{
+	return -EOPNOTSUPP;
+}
+
+void mk_force_stop_cpu(mk_phys_cpu_t phys_cpu)
+{
+	pr_warn_once("RISC-V multikernel force-stop is not implemented\n");
+}
+
+int mk_arch_spawn_instance(struct kimage *image, struct mk_instance *instance,
+			   int cpu)
+{
+	return -EOPNOTSUPP;
+}
+
+int mk_arch_release_instance(struct mk_instance *instance)
+{
+	return 0;
+}
+
+int mk_arch_confirm_parked(struct mk_instance *instance,
+			   mk_phys_cpu_t phys_cpu)
+{
+	return -EOPNOTSUPP;
+}
+
+int mk_repark_instance_to_host(struct mk_instance *instance)
+{
+	return -EOPNOTSUPP;
+}
+
+int mk_repark_cpu_to_instance(struct mk_instance *instance,
+			      mk_phys_cpu_t phys_cpu)
+{
+	return -EOPNOTSUPP;
+}
+
+int mk_repark_cpu_to_host(struct mk_instance *instance,
+			  mk_phys_cpu_t phys_cpu)
+{
+	return -EOPNOTSUPP;
+}

arch/x86/Kconfig

@@ -2026,6 +2026,9 @@ config ARCH_SUPPORTS_MULTIKERNEL
 config ARCH_HAS_MK_HOST_PARK
 	def_bool X86_64
 
+config ARCH_HAS_MK_POOL_STATE
+	def_bool X86_64
+
 config ARCH_SUPPORTS_CRASH_DUMP
 	def_bool X86_64 || (X86_32 && HIGHMEM)
 

include/linux/multikernel.h

@@ -1029,6 +1029,8 @@ void *mk_instance_ctrl_alloc(struct mk_instance *instance, size_t size,
  *  - CONFIG_ARCH_HAS_MK_HOST_PARK and the host park functions declared
  *    with the pool chunk API above, for architectures that park CPUs in
  *    software rather than in firmware.
+ *  - CONFIG_ARCH_HAS_MK_POOL_STATE, if the architecture can park an
+ *    offlined CPU for reassignment to another kernel instance.
  */
 
 /* Doorbell for the message ring: IPI a CPU owned by another kernel */

kernel/multikernel/Kconfig

@@ -10,10 +10,18 @@ config ARCH_SUPPORTS_MULTIKERNEL
 config ARCH_HAS_MK_HOST_PARK
 	bool
 
+# The arch can park an offlined CPU in a state from which multikernel can
+# reassign it to another kernel instance.
+config ARCH_HAS_MK_POOL_STATE
+	bool
+
 config MULTIKERNEL
 	bool "Multikernel support"
 	depends on ARCH_SUPPORTS_MULTIKERNEL
+	depends on CONTIG_ALLOC
 	depends on KEXEC_CORE
+	depends on MEMORY_HOTPLUG
+	depends on MEMORY_HOTREMOVE
 	select LIBFDT
 	help
 	  Enable multikernel support, which allows running multiple kernel

kernel/multikernel/hotplug.c

@@ -192,6 +192,10 @@ static int mk_do_cpu_remove(mk_phys_cpu_t cpu_id)
 	int ret;
 	struct mk_hotplug_op *op;
 
+	/* Reject unsupported pool transitions before offlining the CPU. */
+	if (!IS_ENABLED(CONFIG_ARCH_HAS_MK_POOL_STATE))
+		return -EOPNOTSUPP;
+
 	logical_cpu = mk_cpu_to_logical(cpu_id);
 	if (logical_cpu < 0) {
 		pr_err("Multikernel hotplug: Physical CPU %llu not found\n", cpu_id);