xz: adjust arch-specific options for better kernel compression

Use LZMA2 options that match the arch-specific alignment of instructions.
This change reduces compressed kernel size 0-2 % depending on the arch.
On 1-byte-aligned x86 it makes no difference and on 4-byte-aligned archs
it helps the most.

Use the ARM-Thumb filter for ARM-Thumb2 kernels.  This reduces compressed
kernel size about 5 %.[1] Previously such kernels were compressed using
the ARM filter which didn't do anything useful with ARM-Thumb2 code.

Add BCJ filter support for ARM64 and RISC-V.  Compared to unfiltered XZ or
plain LZMA, the compressed kernel size is reduced about 5 % on ARM64 and 7
% on RISC-V.  A new enough version of the xz tool is required: 5.4.0 for
ARM64 and 5.6.0 for RISC-V.  With an old xz version, a message is printed
to standard error and the kernel is compressed without the filter.

Update lib/decompress_unxz.c to match the changes to xz_wrap.sh.

Update the CONFIG_KERNEL_XZ help text in init/Kconfig:
  - Add the RISC-V and ARM64 filters.
  - Clarify that the PowerPC filter is for big endian only.
  - Omit IA-64.

Link: https://lore.kernel.org/lkml/1637379771-39449-1-git-send-email-zhongjubin@huawei.com/
Link: https://lkml.kernel.org/r/20240721133633.47721-15-lasse.collin@tukaani.org
Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
Reviewed-by: Sam James <sam@gentoo.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Jubin Zhong <zhongjubin@huawei.com>
Cc: Jules Maselbas <jmaselbas@zdiv.net>
Cc: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Rui Li <me@lirui.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/init/Kconfig b/init/Kconfig
index 5783a0b8751726ad97189d786dca9ee43cb8cd3c..583cb07176a97fe52362e9b26e1c5d284661b513 100644 (file)
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -310,8 +310,9 @@ config KERNEL_XZ
          BCJ filters which can improve compression ratio of executable
          code. The size of the kernel is about 30% smaller with XZ in
          comparison to gzip. On architectures for which there is a BCJ
-         filter (i386, x86_64, ARM, IA-64, PowerPC, and SPARC), XZ
-         will create a few percent smaller kernel than plain LZMA.
+         filter (i386, x86_64, ARM, ARM64, RISC-V, big endian PowerPC,
+         and SPARC), XZ will create a few percent smaller kernel than
+         plain LZMA.
 
          The speed is about the same as with LZMA: The decompression
          speed of XZ is better than that of bzip2 but worse than gzip

diff --git a/lib/decompress_unxz.c b/lib/decompress_unxz.c
index 46aa3be13fc5371332de258fc9f68285e61f7a59..cae00395d7a62cc865cf02c96e5399feb063fffe 100644 (file)
--- a/lib/decompress_unxz.c
+++ b/lib/decompress_unxz.c
@@ -126,11 +126,21 @@
 #ifdef CONFIG_X86
 #      define XZ_DEC_X86
 #endif
-#ifdef CONFIG_PPC
+#if defined(CONFIG_PPC) && defined(CONFIG_CPU_BIG_ENDIAN)
 #      define XZ_DEC_POWERPC
 #endif
 #ifdef CONFIG_ARM
-#      define XZ_DEC_ARM
+#      ifdef CONFIG_THUMB2_KERNEL
+#              define XZ_DEC_ARMTHUMB
+#      else
+#              define XZ_DEC_ARM
+#      endif
+#endif
+#ifdef CONFIG_ARM64
+#      define XZ_DEC_ARM64
+#endif
+#ifdef CONFIG_RISCV
+#      define XZ_DEC_RISCV
 #endif
 #ifdef CONFIG_SPARC
 #      define XZ_DEC_SPARC

diff --git a/scripts/xz_wrap.sh b/scripts/xz_wrap.sh
index c8c36441ab706e5bd739e1d74921614db26cb593..f19369687030656577c2b4cfa129f66cdc806d59 100755 (executable)
--- a/scripts/xz_wrap.sh
+++ b/scripts/xz_wrap.sh
@@ -6,14 +6,146 @@
 #
 # Author: Lasse Collin <lasse.collin@tukaani.org>
 
+# This has specialized settings for the following archs. However,
+# XZ-compressed kernel isn't currently supported on every listed arch.
+#
+#   Arch        Align   Notes
+#   arm          2/4    ARM and ARM-Thumb2
+#   arm64         4
+#   csky          2
+#   loongarch     4
+#   mips         2/4    MicroMIPS is 2-byte aligned
+#   parisc        4
+#   powerpc       4     Uses its own wrapper for compressors instead of this.
+#   riscv        2/4
+#   s390          2
+#   sh            2
+#   sparc         4
+#   x86           1
+
+# A few archs use 2-byte or 4-byte aligned instructions depending on
+# the kernel config. This function is used to check if the relevant
+# config option is set to "y".
+is_enabled()
+{
+       grep -q "^$1=y$" include/config/auto.conf
+}
+
+# XZ_VERSION is needed to disable features that aren't available in
+# old XZ Utils versions.
+XZ_VERSION=$($XZ --robot --version) || exit
+XZ_VERSION=$(printf '%s\n' "$XZ_VERSION" | sed -n 's/^XZ_VERSION=//p')
+
+# Assume that no BCJ filter is available.
 BCJ=
-LZMA2OPTS=
 
+# Set the instruction alignment to 1, 2, or 4 bytes.
+#
+# Set the BCJ filter if one is available.
+# It must match the #ifdef usage in lib/decompress_unxz.c.
 case $SRCARCH in
-       x86)            BCJ=--x86 ;;
-       powerpc)        BCJ=--powerpc ;;
-       arm)            BCJ=--arm ;;
-       sparc)          BCJ=--sparc ;;
+       arm)
+               if is_enabled CONFIG_THUMB2_KERNEL; then
+                       ALIGN=2
+                       BCJ=--armthumb
+               else
+                       ALIGN=4
+                       BCJ=--arm
+               fi
+               ;;
+
+       arm64)
+               ALIGN=4
+
+               # ARM64 filter was added in XZ Utils 5.4.0.
+               if [ "$XZ_VERSION" -ge 50040002 ]; then
+                       BCJ=--arm64
+               else
+                       echo "$0: Upgrading to xz >= 5.4.0" \
+                               "would enable the ARM64 filter" \
+                               "for better compression" >&2
+               fi
+               ;;
+
+       csky)
+               ALIGN=2
+               ;;
+
+       loongarch)
+               ALIGN=4
+               ;;
+
+       mips)
+               if is_enabled CONFIG_CPU_MICROMIPS; then
+                       ALIGN=2
+               else
+                       ALIGN=4
+               fi
+               ;;
+
+       parisc)
+               ALIGN=4
+               ;;
+
+       powerpc)
+               ALIGN=4
+
+               # The filter is only for big endian instruction encoding.
+               if is_enabled CONFIG_CPU_BIG_ENDIAN; then
+                       BCJ=--powerpc
+               fi
+               ;;
+
+       riscv)
+               if is_enabled CONFIG_RISCV_ISA_C; then
+                       ALIGN=2
+               else
+                       ALIGN=4
+               fi
+
+               # RISC-V filter was added in XZ Utils 5.6.0.
+               if [ "$XZ_VERSION" -ge 50060002 ]; then
+                       BCJ=--riscv
+               else
+                       echo "$0: Upgrading to xz >= 5.6.0" \
+                               "would enable the RISC-V filter" \
+                               "for better compression" >&2
+               fi
+               ;;
+
+       s390)
+               ALIGN=2
+               ;;
+
+       sh)
+               ALIGN=2
+               ;;
+
+       sparc)
+               ALIGN=4
+               BCJ=--sparc
+               ;;
+
+       x86)
+               ALIGN=1
+               BCJ=--x86
+               ;;
+
+       *)
+               echo "$0: Arch-specific tuning is missing for '$SRCARCH'" >&2
+
+               # Guess 2-byte-aligned instructions. Guessing too low
+               # should hurt less than guessing too high.
+               ALIGN=2
+               ;;
+esac
+
+# Select the LZMA2 options matching the instruction alignment.
+case $ALIGN in
+       1)  LZMA2OPTS= ;;
+       2)  LZMA2OPTS=lp=1 ;;
+       4)  LZMA2OPTS=lp=2,lc=2 ;;
+       *)  echo "$0: ALIGN wrong or missing" >&2; exit 1 ;;
 esac
 
 # Use single-threaded mode because it compresses a little better