[RFC 1/6] RISC-V: Avoid using per cpu array for ordered booting

[Atish Patra]

From: Atish Patra <atishp at rivosinc.com>

Currently both order booting and spinwait approach uses a per cpu
array to update stack & task pointer. This approach will not work for the
following cases.
1. If NR_CPUs are configured to be less than highest hart id.
2. A platform has sparse hartid.

This issue can be fixed for ordered booting as the booting cpu brings up
one cpu at a time using SBI HSM extension which has opaque parameter
that is unused until now.

Introduce a common secondary boot data structure that can store the stack
and task pointer. Secondary harts will use this data while booting up
to setup the sp & tp.

Signed-off-by: Atish Patra <atishp at rivosinc.com>
---
 arch/riscv/include/asm/cpu_ops_sbi.h | 28 ++++++++++++++++++++++++++++
 arch/riscv/kernel/cpu_ops_sbi.c      | 23 ++++++++++++++++++++---
 arch/riscv/kernel/head.S             | 19 ++++++++++---------
 3 files changed, 58 insertions(+), 12 deletions(-)
 create mode 100644 arch/riscv/include/asm/cpu_ops_sbi.h

diff --git a/arch/riscv/include/asm/cpu_ops_sbi.h b/arch/riscv/include/asm/cpu_ops_sbi.h
new file mode 100644
index 000000000000..ccb9a6d30486
--- /dev/null
+++ b/arch/riscv/include/asm/cpu_ops_sbi.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2021 by Rivos Inc.
+ */
+#ifndef __ASM_CPU_OPS_SBI_H
+#define __ASM_CPU_OPS_SBI_H
+
+#ifndef __ASSEMBLY__
+#include <linux/init.h>
+#include <linux/sched.h>
+#include <linux/threads.h>
+
+/**
+ * struct sbi_hart_boot_data - Hart specific boot used during booting and
+ *			       cpu hotplug.
+ * @task_ptr: A pointer to the hart specific tp
+ * @stack_ptr: A pointer to the hart specific sp
+ */
+struct sbi_hart_boot_data {
+	void *task_ptr;
+	void *stack_ptr;
+};
+#endif
+
+#define SBI_HART_BOOT_TASK_PTR_OFFSET (0x00)
+#define SBI_HART_BOOT_STACK_PTR_OFFSET RISCV_SZPTR
+
+#endif /* ifndef __ASM_CPU_OPS_H */
diff --git a/arch/riscv/kernel/cpu_ops_sbi.c b/arch/riscv/kernel/cpu_ops_sbi.c
index 685fae72b7f5..2e7a9dd9c2a7 100644
--- a/arch/riscv/kernel/cpu_ops_sbi.c
+++ b/arch/riscv/kernel/cpu_ops_sbi.c
@@ -7,13 +7,22 @@
 
 #include <linux/init.h>
 #include <linux/mm.h>
+#include <linux/sched/task_stack.h>
 #include <asm/cpu_ops.h>
+#include <asm/cpu_ops_sbi.h>
 #include <asm/sbi.h>
 #include <asm/smp.h>
 
 extern char secondary_start_sbi[];
 const struct cpu_operations cpu_ops_sbi;
 
+/*
+ * Ordered booting via HSM brings one cpu at a time. However, cpu hotplug can
+ * be invoked from multiple threads in paralle. Define a per cpu data
+ * to handle that.
+ */
+DEFINE_PER_CPU(struct sbi_hart_boot_data, boot_data);
+
 static int sbi_hsm_hart_start(unsigned long hartid, unsigned long saddr,
 			      unsigned long priv)
 {
@@ -58,9 +67,17 @@ static int sbi_cpu_start(unsigned int cpuid, struct task_struct *tidle)
 	int rc;
 	unsigned long boot_addr = __pa_symbol(secondary_start_sbi);
 	int hartid = cpuid_to_hartid_map(cpuid);
-
-	cpu_update_secondary_bootdata(cpuid, tidle);
-	rc = sbi_hsm_hart_start(hartid, boot_addr, 0);
+	unsigned long hsm_data;
+	struct sbi_hart_boot_data *bdata = &per_cpu(boot_data, cpuid);
+
+	/* Make sure tidle is updated */
+	smp_mb();
+	bdata->task_ptr = tidle;
+	bdata->stack_ptr = task_stack_page(tidle) + THREAD_SIZE;
+	/* Make sure boot data is updated */
+	smp_mb();
+	hsm_data = __pa(bdata);
+	rc = sbi_hsm_hart_start(hartid, boot_addr, hsm_data);
 
 	return rc;
 }
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index f52f01ecbeea..40d4c625513c 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -11,6 +11,7 @@
 #include <asm/page.h>
 #include <asm/pgtable.h>
 #include <asm/csr.h>
+#include <asm/cpu_ops_sbi.h>
 #include <asm/hwcap.h>
 #include <asm/image.h>
 #include "efi-header.S"
@@ -167,15 +168,15 @@ secondary_start_sbi:
 	la a3, .Lsecondary_park
 	csrw CSR_TVEC, a3
 
-	slli a3, a0, LGREG
-	la a4, __cpu_up_stack_pointer
-	XIP_FIXUP_OFFSET a4
-	la a5, __cpu_up_task_pointer
-	XIP_FIXUP_OFFSET a5
-	add a4, a3, a4
-	add a5, a3, a5
-	REG_L sp, (a4)
-	REG_L tp, (a5)
+	/* a0 contains the hartid & a1 contains boot data */
+	li a2, SBI_HART_BOOT_TASK_PTR_OFFSET
+	XIP_FIXUP_OFFSET a2
+	add a2, a2, a1
+	REG_L tp, (a2)
+	li a3, SBI_HART_BOOT_STACK_PTR_OFFSET
+	XIP_FIXUP_OFFSET a3
+	add a3, a3, a1
+	REG_L sp, (a3)
 
 	.global secondary_start_common
 secondary_start_common:
-- 
2.33.1