Re: [PATCH 2/2] MIPS: Add support for ZSTD-compressed kernels

[Nick Terrell <terrelln@fb.com>]

> On Aug 24, 2020, at 2:02 PM, Paul Cercueil <paul@crapouillou.net> wrote:
> 
> Hi Nick,
> 
> Le lun. 24 août 2020 à 19:51, Nick Terrell <terrelln@fb.com> a écrit :
>>> On Aug 21, 2020, at 9:29 AM, Paul Cercueil <paul@crapouillou.net> wrote:
>>> 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.
>>> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
>>> ---
>>> arch/mips/Kconfig                      |  1 +
>>> arch/mips/boot/compressed/Makefile     |  1 +
>>> arch/mips/boot/compressed/decompress.c |  4 ++++
>>> arch/mips/boot/compressed/string.c     | 16 ++++++++++++++++
>>> 4 files changed, 22 insertions(+)
>>> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
>>> index c95fa3a2484c..b9d7c4249dc9 100644
>>> --- a/arch/mips/Kconfig
>>> +++ b/arch/mips/Kconfig
>>> @@ -1890,6 +1890,7 @@ 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
>>> diff --git a/arch/mips/boot/compressed/Makefile b/arch/mips/boot/compressed/Makefile
>>> index 6e56caef69f0..86ddc6fc16f4 100644
>>> --- a/arch/mips/boot/compressed/Makefile
>>> +++ b/arch/mips/boot/compressed/Makefile
>>> @@ -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)    = zstd
>> You can use zstd22 here. It will give you slightly better compression
>> without any additional memory usage. Also, you should add
>> -D__DISABLE_EXPORTS to the KBUILD_CFLAGS like x86 does [1].
> 
> Indeed, it's 0.01% smaller :)
> 
> What is __DISABLE_EXPORTS for?

It disables the EXPORT_SYMBOL() macros inside of lib/zstd/decompress.c.
On x86 the kernel won’t boot with these defined. Other decompressors hide
them if the STATIC macro is defined, but zstd uses this method, which was
added somewhat recently.

-Nick

> -Paul
> 
>> [1] https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/Makefile
>> -Nick
>>> targets += vmlinux.bin.z
>>> $(obj)/vmlinux.bin.z: $(obj)/vmlinux.bin FORCE
>>> diff --git a/arch/mips/boot/compressed/decompress.c b/arch/mips/boot/compressed/decompress.c
>>> index 88f5d637b1c4..c61c641674e6 100644
>>> --- a/arch/mips/boot/compressed/decompress.c
>>> +++ b/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)
>>> diff --git a/arch/mips/boot/compressed/string.c b/arch/mips/boot/compressed/string.c
>>> index 43beecc3587c..ab95722ec0c9 100644
>>> --- a/arch/mips/boot/compressed/string.c
>>> +++ b/arch/mips/boot/compressed/string.c
>>> @@ -27,3 +27,19 @@ void *memset(void *s, int c, size_t n)
>>> 		ss[i] = c;
>>> 	return s;
>>> }
>>> +
>>> +void *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;
>>> +}
>>> --
>>> 2.28.0