[PATCH kernel] powerpc/llvm/lto: Allow LLVM LTO builds
Nick Desaulniers
ndesaulniers at google.com
Wed May 4 07:21:21 AEST 2022
On Thu, Apr 28, 2022 at 11:46 PM Alexey Kardashevskiy <aik at ozlabs.ru> wrote:
>
> This enables LTO_CLANG builds on POWER with the upstream version of
> LLVM.
>
> LTO optimizes the output vmlinux binary and this may affect the FTP
> alternative section if alt branches use "bc" (Branch Conditional) which
> is limited by 16 bit offsets. This shows up in errors like:
>
> ld.lld: error: InputSection too large for range extension thunk vmlinux.o:(__ftr_alt_97+0xF0)
>
> This works around the issue by replacing "bc" in FTR_SECTION_ELSE with
> "b" which allows 26 bit offsets.
>
> This catches the problem instructions in vmlinux.o before it LTO'ed:
>
> $ objdump -d -M raw -j __ftr_alt_97 vmlinux.o | egrep '\S+\s*\<bc\>'
> 30: 00 00 82 40 bc 4,eq,30 <__ftr_alt_97+0x30>
> f0: 00 00 82 40 bc 4,eq,f0 <__ftr_alt_97+0xf0>
>
> This allows LTO builds for ppc64le_defconfig plus LTO options.
> Note that DYNAMIC_FTRACE/FUNCTION_TRACER is not supported by LTO builds
> but this is not POWERPC-specific.
$ ARCH=powerpc make LLVM=1 -j72 ppc64le_defconfig
$ ARCH=powerpc make LLVM=1 -j72 menuconfig
<disable FTRACE, enable LTO_CLANG_THIN>
$ ARCH=powerpc make LLVM=1 -j72
...
VDSO64L arch/powerpc/kernel/vdso/vdso64.so.dbg
/usr/bin/powerpc64le-linux-gnu-ld:
/android0/llvm-project/llvm/build/bin/../lib/LLVMgold.so: error
loading plugin:
/android0/llvm-project/llvm/build/bin/../lib/LLVMgold.so: cannot open
shared object file: No such file or directory
clang-15: error: linker command failed with exit code 1 (use -v to see
invocation)
make[1]: *** [arch/powerpc/kernel/vdso/Makefile:67:
arch/powerpc/kernel/vdso/vdso64.so.dbg] Error 1
Looks like LLD isn't being invoked correctly to link the vdso.
Probably need to revisit
https://lore.kernel.org/lkml/20200901222523.1941988-1-ndesaulniers@google.com/
How were you working around this issue? Perhaps you built clang to
default to LLD? (there's a cmake option for that)
Perhaps for now I should just send:
```
diff --git a/arch/powerpc/kernel/vdso/Makefile
b/arch/powerpc/kernel/vdso/Makefile
index 954974287ee7..8762e6513683 100644
--- a/arch/powerpc/kernel/vdso/Makefile
+++ b/arch/powerpc/kernel/vdso/Makefile
@@ -55,6 +55,11 @@ AS32FLAGS := -D__VDSO32__ -s
CC64FLAGS := -Wl,-soname=linux-vdso64.so.1
AS64FLAGS := -D__VDSO64__ -s
+ifneq ($(LLVM),)
+CC32FLAGS += -fuse-ld=lld
+CC64FLAGS += -fuse-ld=lld
+endif
+
targets += vdso32.lds
CPPFLAGS_vdso32.lds += -P -C -Upowerpc
targets += vdso64.lds
```
>
> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
> ---
>
> Note 1:
> This is further development of
> https://lore.kernel.org/all/20220211023125.1790960-1-aik@ozlabs.ru/T/
>
> Note 2:
> CONFIG_ZSTD_COMPRESS and CONFIG_ZSTD_DECOMPRESS must be both "m" or "y"
> or it won't link. For details:
> https://lore.kernel.org/lkml/20220428043850.1706973-1-aik@ozlabs.ru/T/
Yeah, I just hit this:
```
LTO vmlinux.o
LLVM ERROR: Function Import: link error: linking module flags 'Code
Model': IDs have conflicting values in
'lib/built-in.a(entropy_common.o at 5782)' and
'lib/built-in.a(zstd_decompress_block.o at 6202)'
PLEASE submit a bug report to
https://github.com/llvm/llvm-project/issues/ and include the crash
backtrace.
LLVM ERROR: Failed to rename temporary file
.thinlto-cache/Thin-96f93f.tmp.o to
.thinlto-cache/llvmcache-A5B351EA452D46A86980E29C78B7260673348AAF: No
such file or directory
scripts/link-vmlinux.sh: line 76: 1240312 Aborted
${LD} ${KBUILD_LDFLAGS} -r -o ${1} ${lds} ${objects}
make: *** [Makefile:1158: vmlinux] Error 134
```
These two configs aren't easily modified in menuconfig. Perhaps you
could find the concise set of configs that need to be disabled for
this to be buildable? At least so others can test more easily, or even
so we can update Kconfig checks.
> ---
> arch/powerpc/Kconfig | 2 ++
> arch/powerpc/kernel/exceptions-64s.S | 4 +++-
> arch/powerpc/lib/copyuser_64.S | 3 ++-
> arch/powerpc/lib/feature-fixups-test.S | 3 +--
> arch/powerpc/lib/memcpy_64.S | 3 ++-
> 5 files changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 174edabb74fa..e2c7b5c1d0a6 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -158,6 +158,8 @@ config PPC
> select ARCH_WANT_IRQS_OFF_ACTIVATE_MM
> select ARCH_WANT_LD_ORPHAN_WARN
> select ARCH_WEAK_RELEASE_ACQUIRE
> + select ARCH_SUPPORTS_LTO_CLANG
> + select ARCH_SUPPORTS_LTO_CLANG_THIN
> select BINFMT_ELF
> select BUILDTIME_TABLE_SORT
> select CLONE_BACKWARDS
> diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
> index b66dd6f775a4..5b783bd51260 100644
> --- a/arch/powerpc/kernel/exceptions-64s.S
> +++ b/arch/powerpc/kernel/exceptions-64s.S
> @@ -476,9 +476,11 @@ DEFINE_FIXED_SYMBOL(\name\()_common_real, text)
> .if IHSRR_IF_HVMODE
> BEGIN_FTR_SECTION
> bne masked_Hinterrupt
> + b 4f
> FTR_SECTION_ELSE
> - bne masked_interrupt
> ALT_FTR_SECTION_END_IFSET(CPU_FTR_HVMODE | CPU_FTR_ARCH_206)
> + bne masked_interrupt
> +4:
> .elseif IHSRR
> bne masked_Hinterrupt
> .else
> diff --git a/arch/powerpc/lib/copyuser_64.S b/arch/powerpc/lib/copyuser_64.S
> index db8719a14846..d07f95eebc65 100644
> --- a/arch/powerpc/lib/copyuser_64.S
> +++ b/arch/powerpc/lib/copyuser_64.S
> @@ -75,10 +75,11 @@ _GLOBAL(__copy_tofrom_user_base)
> * set is Power6.
> */
> test_feature = (SELFTEST_CASE == 1)
> + beq .Ldst_aligned
> BEGIN_FTR_SECTION
> nop
> FTR_SECTION_ELSE
> - bne .Ldst_unaligned
> + b .Ldst_unaligned
> ALT_FTR_SECTION_END(CPU_FTR_UNALIGNED_LD_STD | CPU_FTR_CP_USE_DCBTZ, \
> CPU_FTR_UNALIGNED_LD_STD)
> .Ldst_aligned:
> diff --git a/arch/powerpc/lib/feature-fixups-test.S b/arch/powerpc/lib/feature-fixups-test.S
> index 480172fbd024..2751e42a9fd7 100644
> --- a/arch/powerpc/lib/feature-fixups-test.S
> +++ b/arch/powerpc/lib/feature-fixups-test.S
> @@ -145,7 +145,6 @@ BEGIN_FTR_SECTION
> FTR_SECTION_ELSE
> 2: or 2,2,2
> PPC_LCMPI r3,1
> - beq 3f
> blt 2b
> b 3f
> b 1b
> @@ -160,10 +159,10 @@ globl(ftr_fixup_test6_expected)
> 1: or 1,1,1
> 2: or 2,2,2
> PPC_LCMPI r3,1
> - beq 3f
> blt 2b
> b 3f
> b 1b
> + nop
> 3: or 1,1,1
> or 2,2,2
> or 3,3,3
> diff --git a/arch/powerpc/lib/memcpy_64.S b/arch/powerpc/lib/memcpy_64.S
> index 016c91e958d8..286c7e2d0883 100644
> --- a/arch/powerpc/lib/memcpy_64.S
> +++ b/arch/powerpc/lib/memcpy_64.S
> @@ -50,10 +50,11 @@ ALT_FTR_SECTION_END_IFCLR(CPU_FTR_VMX_COPY)
> At the time of writing the only CPU that has this combination of bits
> set is Power6. */
> test_feature = (SELFTEST_CASE == 1)
> + beq .ldst_aligned
> BEGIN_FTR_SECTION
> nop
> FTR_SECTION_ELSE
> - bne .Ldst_unaligned
> + b .Ldst_unaligned
> ALT_FTR_SECTION_END(CPU_FTR_UNALIGNED_LD_STD | CPU_FTR_CP_USE_DCBTZ, \
> CPU_FTR_UNALIGNED_LD_STD)
> .Ldst_aligned:
> --
> 2.30.2
>
--
Thanks,
~Nick Desaulniers
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