LoongArch: Add kdump support

This patch adds support for kdump. In kdump case the normal kernel will
reserve a region for the crash kernel and jump there on panic.

Arch-specific functions are added to allow for implementing a crash dump
file interface, /proc/vmcore, which can be viewed as a ELF file.

A user-space tool, such as kexec-tools, is responsible for allocating a
separate region for the core's ELF header within the crash kdump kernel
memory and filling it in when executing kexec_load().

Then, its location will be advertised to the crash dump kernel via a
command line argument "elfcorehdr=", and the crash dump kernel will
preserve this region for later use with arch_reserve_vmcore() at boot
time.

At the same time, the crash kdump kernel is also limited within the
"crashkernel" area via a command line argument "mem=", so as not to
destroy the original kernel dump data.

In the crash dump kernel environment, /proc/vmcore is used to access the
primary kernel's memory with copy_oldmem_page().

I tested kdump on LoongArch machines (Loongson-3A5000) and it works as
expected (suggested crashkernel parameter is "crashkernel=512M@2560M"),
you may test it by triggering a crash through /proc/sysrq-trigger:

 $ sudo kexec -p /boot/vmlinux-kdump --reuse-cmdline --append="nr_cpus=1"
 # echo c > /proc/sysrq-trigger

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

1 files changed, 23 insertions, 0 deletions

diff --git a/arch/loongarch/kernel/crash_dump.c b/arch/loongarch/kernel/crash_dump.c
new file mode 100644
index 000000000000..e559307c1092
--- /dev/null
+++ b/arch/loongarch/kernel/crash_dump.c
@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/crash_dump.h>
+#include <linux/io.h>
+#include <linux/uio.h>
+
+ssize_t copy_oldmem_page(struct iov_iter *iter, unsigned long pfn,
+			 size_t csize, unsigned long offset)
+{
+	void *vaddr;
+
+	if (!csize)
+		return 0;
+
+	vaddr = memremap(__pfn_to_phys(pfn), PAGE_SIZE, MEMREMAP_WB);
+	if (!vaddr)
+		return -ENOMEM;
+
+	csize = copy_to_iter(vaddr + offset, csize, iter);
+
+	memunmap(vaddr);
+
+	return csize;
+}