[PATCH] RISC-V: Add fast call path of crash_kexec()

[Xianting Tian]

Currently, almost all archs (x86, arm64, mips...) support fast call
of crash_kexec() when "regs && kexec_should_crash()" is true. But
RISC-V not, it can only enter crash system via panic(). However panic()
doesn't pass the regs of the real accident scene to crash_kexec(),
it caused we can't get accurate backtrace via gdb,
	$ riscv64-linux-gnu-gdb vmlinux vmcore
	Reading symbols from vmlinux...
	[New LWP 95]
	#0  console_unlock () at kernel/printk/printk.c:2557
	2557                    if (do_cond_resched)
	(gdb) bt
	#0  console_unlock () at kernel/printk/printk.c:2557
	#1  0x0000000000000000 in ?? ()

With the patch we can get the accurate backtrace,
	$ riscv64-linux-gnu-gdb vmlinux vmcore
	Reading symbols from vmlinux...
	[New LWP 95]
	#0  0xffffffe00063a4e0 in test_thread (data=<optimized out>) at drivers/virtio/virtio_mmio.c:806
	806             *(int *)p = 0xdead;
	(gdb)
	(gdb) bt
	#0  0xffffffe00063a4e0 in test_thread (data=<optimized out>) at drivers/virtio/virtio_mmio.c:806
	#1  0x0000000000000000 in ?? ()

Test code to produce NULL address dereference,
	+extern int panic_on_oops;
	+static struct task_struct *k;
	+static int test_thread(void *data) {
	+
	+       void *p = NULL;
	+
	+       while (!panic_on_oops)
	+               msleep(2000);
	+
	+       *(int *)p = 0xdead;
	+
	+       return 0;
	+}
	+
	 static int __init virtio_mmio_init(void)
	 {
	+       k = kthread_run(test_thread, NULL, "test_thread");
	+       if (IS_ERR(k))
	+               pr_err("Couldn't create test kthread\n");
	+
	        return platform_driver_register(&virtio_mmio_driver);
	 }

Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
---
 arch/riscv/kernel/traps.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index fe92e119e6a3..e666ebfa2a64 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -16,6 +16,7 @@
 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/irq.h>
+#include <linux/kexec.h>
 
 #include <asm/asm-prototypes.h>
 #include <asm/bug.h>
@@ -44,6 +45,9 @@ void die(struct pt_regs *regs, const char *str)
 
 	ret = notify_die(DIE_OOPS, str, regs, 0, regs->cause, SIGSEGV);
 
+	if (regs && kexec_should_crash(current))
+		crash_kexec(regs);
+
 	bust_spinlocks(0);
 	add_taint(TAINT_DIE, LOCKDEP_NOW_UNRELIABLE);
 	spin_unlock_irq(&die_lock);
-- 
2.17.1