MIPS: Add support for ZSTD-compressed kernels

Add support for self-extracting kernels with a ZSTD compression.

Tested on a kernel for the GCW-Zero, it allows to reduce the size of the
kernel file from 4.1 MiB with gzip to 3.5 MiB with ZSTD, and boots just
as fast.

Compressed kernels are now also compiled with -D__DISABLE_EXPORTS in
order to disable the EXPORT_SYMBOL() macros inside of
lib/zstd/decompress.c.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/Kconfig

@@ -1860,6 +1860,7 @@ config SYS_SUPPORTS_ZBOOT
 	select HAVE_KERNEL_LZMA
 	select HAVE_KERNEL_LZO
 	select HAVE_KERNEL_XZ
+	select HAVE_KERNEL_ZSTD
 
 config SYS_SUPPORTS_ZBOOT_UART16550
 	bool

arch/mips/boot/compressed/Makefile

@@ -22,7 +22,7 @@ KBUILD_CFLAGS := $(filter-out -pg, $(KBUILD_CFLAGS))
 
 KBUILD_CFLAGS := $(filter-out -fstack-protector, $(KBUILD_CFLAGS))
 
-KBUILD_CFLAGS := $(KBUILD_CFLAGS) -D__KERNEL__ \
+KBUILD_CFLAGS := $(KBUILD_CFLAGS) -D__KERNEL__ -D__DISABLE_EXPORTS \
 	-DBOOT_HEAP_SIZE=$(BOOT_HEAP_SIZE) -D"VMLINUX_LOAD_ADDRESS_ULL=$(VMLINUX_LOAD_ADDRESS)ull"
 
 KBUILD_AFLAGS := $(KBUILD_AFLAGS) -D__ASSEMBLY__ \
@@ -70,6 +70,7 @@ tool_$(CONFIG_KERNEL_LZ4)     = lz4
 tool_$(CONFIG_KERNEL_LZMA)    = lzma
 tool_$(CONFIG_KERNEL_LZO)     = lzo
 tool_$(CONFIG_KERNEL_XZ)      = xzkern
+tool_$(CONFIG_KERNEL_ZSTD)    = zstd22
 
 targets += vmlinux.bin.z
 $(obj)/vmlinux.bin.z: $(obj)/vmlinux.bin FORCE

arch/mips/boot/compressed/decompress.c

@@ -72,6 +72,10 @@ void error(char *x)
 #include "../../../../lib/decompress_unxz.c"
 #endif
 
+#ifdef CONFIG_KERNEL_ZSTD
+#include "../../../../lib/decompress_unzstd.c"
+#endif
+
 const unsigned long __stack_chk_guard = 0x000a0dff;
 
 void __stack_chk_fail(void)

arch/mips/boot/compressed/string.c

@@ -5,6 +5,7 @@
  * Very small subset of simple string routines
  */
 
+#include <linux/compiler_attributes.h>
 #include <linux/types.h>
 
 void *memcpy(void *dest, const void *src, size_t n)
@@ -27,3 +28,19 @@ void *memset(void *s, int c, size_t n)
 		ss[i] = c;
 	return s;
 }
+
+void * __weak memmove(void *dest, const void *src, size_t n)
+{
+	unsigned int i;
+	const char *s = src;
+	char *d = dest;
+
+	if ((uintptr_t)dest < (uintptr_t)src) {
+		for (i = 0; i < n; i++)
+			d[i] = s[i];
+	} else {
+		for (i = n; i > 0; i--)
+			d[i - 1] = s[i - 1];
+	}
+	return dest;
+}