[RFC 3/6] RISC-V: Use __cpu_up_stack/task_pointer only for spinwait method

[Atish Patra]

From: Atish Patra <atishp at rivosinc.com>

The __cpu_up_stack/task_pointer array is only used for spinwait method
now. The per cpu array based lookup is also fragile for platforms with
discontiguous/sparse hartids. The spinwait method is only used for
M-mode Linux or older firmwares without SBI HSM extension. For general
Linux systems, ordered booting method is preferred anyways to support
cpu hotplug and kexec.

Make sure that __cpu_up_stack/task_pointer is only used for spinwait
method. Take this opportunity to rename it to
__cpu_spinwait_stack/task_pointer to emphasize the purpose as well.

Signed-off-by: Atish Patra <atishp at rivosinc.com>
---
 arch/riscv/include/asm/cpu_ops.h     |  2 --
 arch/riscv/kernel/cpu_ops.c          | 16 ----------------
 arch/riscv/kernel/cpu_ops_spinwait.c | 27 ++++++++++++++++++++++++++-
 arch/riscv/kernel/head.S             |  4 ++--
 arch/riscv/kernel/head.h             |  4 ++--
 5 files changed, 30 insertions(+), 23 deletions(-)

diff --git a/arch/riscv/include/asm/cpu_ops.h b/arch/riscv/include/asm/cpu_ops.h
index a8ec3c5c1bd2..134590f1b843 100644
--- a/arch/riscv/include/asm/cpu_ops.h
+++ b/arch/riscv/include/asm/cpu_ops.h
@@ -40,7 +40,5 @@ struct cpu_operations {
 
 extern const struct cpu_operations *cpu_ops[NR_CPUS];
 void __init cpu_set_ops(int cpu);
-void cpu_update_secondary_bootdata(unsigned int cpuid,
-				   struct task_struct *tidle);
 
 #endif /* ifndef __ASM_CPU_OPS_H */
diff --git a/arch/riscv/kernel/cpu_ops.c b/arch/riscv/kernel/cpu_ops.c
index 3f5a38b03044..c1e30f403c3b 100644
--- a/arch/riscv/kernel/cpu_ops.c
+++ b/arch/riscv/kernel/cpu_ops.c
@@ -8,31 +8,15 @@
 #include <linux/of.h>
 #include <linux/string.h>
 #include <linux/sched.h>
-#include <linux/sched/task_stack.h>
 #include <asm/cpu_ops.h>
 #include <asm/sbi.h>
 #include <asm/smp.h>
 
 const struct cpu_operations *cpu_ops[NR_CPUS] __ro_after_init;
 
-void *__cpu_up_stack_pointer[NR_CPUS] __section(".data");
-void *__cpu_up_task_pointer[NR_CPUS] __section(".data");
-
 extern const struct cpu_operations cpu_ops_sbi;
 extern const struct cpu_operations cpu_ops_spinwait;
 
-void cpu_update_secondary_bootdata(unsigned int cpuid,
-				   struct task_struct *tidle)
-{
-	int hartid = cpuid_to_hartid_map(cpuid);
-
-	/* Make sure tidle is updated */
-	smp_mb();
-	WRITE_ONCE(__cpu_up_stack_pointer[hartid],
-		   task_stack_page(tidle) + THREAD_SIZE);
-	WRITE_ONCE(__cpu_up_task_pointer[hartid], tidle);
-}
-
 void __init cpu_set_ops(int cpuid)
 {
 #if IS_ENABLED(CONFIG_RISCV_SBI)
diff --git a/arch/riscv/kernel/cpu_ops_spinwait.c b/arch/riscv/kernel/cpu_ops_spinwait.c
index b2c957bb68c1..9f398eb94f7a 100644
--- a/arch/riscv/kernel/cpu_ops_spinwait.c
+++ b/arch/riscv/kernel/cpu_ops_spinwait.c
@@ -6,11 +6,36 @@
 #include <linux/errno.h>
 #include <linux/of.h>
 #include <linux/string.h>
+#include <linux/sched/task_stack.h>
 #include <asm/cpu_ops.h>
 #include <asm/sbi.h>
 #include <asm/smp.h>
 
 const struct cpu_operations cpu_ops_spinwait;
+void *__cpu_spinwait_stack_pointer[NR_CPUS] __section(".data");
+void *__cpu_spinwait_task_pointer[NR_CPUS] __section(".data");
+
+static void cpu_update_secondary_bootdata(unsigned int cpuid,
+				   struct task_struct *tidle)
+{
+	int hartid = cpuid_to_hartid_map(cpuid);
+
+	/*
+	 * The hartid must be less than NR_CPUS to avoid out-of-bound access
+	 * errors for __cpu_spinwait_stack/task_pointer. That is not always possible
+	 * for platforms with discontiguous hartid numbering scheme. That's why
+	 * spinwait booting is not the recommended approach for any platforms
+	 * and will be removed in future.
+	 */
+	if (hartid == INVALID_HARTID || hartid >= NR_CPUS)
+		return;
+
+	/* Make sure tidle is updated */
+	smp_mb();
+	WRITE_ONCE(__cpu_spinwait_stack_pointer[hartid],
+		   task_stack_page(tidle) + THREAD_SIZE);
+	WRITE_ONCE(__cpu_spinwait_task_pointer[hartid], tidle);
+}
 
 static int spinwait_cpu_prepare(unsigned int cpuid)
 {
@@ -28,7 +53,7 @@ static int spinwait_cpu_start(unsigned int cpuid, struct task_struct *tidle)
 	 * selects the first cpu to boot the kernel and causes the remainder
 	 * of the cpus to spin in a loop waiting for their stack pointer to be
 	 * setup by that main cpu.  Writing to bootdata
-	 * (i.e __cpu_up_stack_pointer) signals to the spinning cpus that they
+	 * (i.e __cpu_spinwait_stack_pointer) signals to the spinning cpus that they
 	 * can continue the boot process.
 	 */
 	cpu_update_secondary_bootdata(cpuid, tidle);
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 40d4c625513c..6f8e99eac6a1 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -347,9 +347,9 @@ clear_bss_done:
 	csrw CSR_TVEC, a3
 
 	slli a3, a0, LGREG
-	la a1, __cpu_up_stack_pointer
+	la a1, __cpu_spinwait_stack_pointer
 	XIP_FIXUP_OFFSET a1
-	la a2, __cpu_up_task_pointer
+	la a2, __cpu_spinwait_task_pointer
 	XIP_FIXUP_OFFSET a2
 	add a1, a3, a1
 	add a2, a3, a2
diff --git a/arch/riscv/kernel/head.h b/arch/riscv/kernel/head.h
index aabbc3ac3e48..5393cca77790 100644
--- a/arch/riscv/kernel/head.h
+++ b/arch/riscv/kernel/head.h
@@ -16,7 +16,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa);
 asmlinkage void __init __copy_data(void);
 #endif
 
-extern void *__cpu_up_stack_pointer[];
-extern void *__cpu_up_task_pointer[];
+extern void *__cpu_spinwait_stack_pointer[];
+extern void *__cpu_spinwait_task_pointer[];
 
 #endif /* __ASM_HEAD_H */
-- 
2.33.1