[PATCH v2 1/2] Makefile: Support building with Clang and LLVM binutils
Bin Meng
bmeng.cn at gmail.com
Fri Jul 9 07:42:40 PDT 2021
On Fri, Jul 9, 2021 at 10:34 PM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
>
> On 9 Jul 2021, at 11:30, Bin Meng <bmeng.cn at gmail.com> wrote:
> >
> > On Fri, Jul 9, 2021 at 3:37 PM Bin Meng <bmeng.cn at gmail.com> wrote:
> >>
> >> On Fri, Jul 9, 2021 at 2:40 PM Bin Meng <bmeng.cn at gmail.com> wrote:
> >>>
> >>> On Fri, Jul 9, 2021 at 11:31 AM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
> >>>>
> >>>> On 9 Jul 2021, at 04:11, Bin Meng <bmeng.cn at gmail.com> wrote:
> >>>>>
> >>>>> On Fri, Jul 9, 2021 at 10:35 AM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
> >>>>>>
> >>>>>> On 9 Jul 2021, at 02:50, Bin Meng <bmeng.cn at gmail.com> wrote:
> >>>>>>>
> >>>>>>> On Fri, Jul 9, 2021 at 1:51 AM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
> >>>>>>>>
> >>>>>>>> This is intended to mirror the Linux kernel. Building with CC=clang will
> >>>>>>>> use Clang as the compiler but default to using the existing binutils.
> >>>>>>>> Building with LLVM=1 will default to using Clang and LLVM binutils.
> >>>>>>>>
> >>>>>>>> Whilst GCC will accept the -N linker option and forward it on to the
> >>>>>>>> linker, Clang will not, and so in order to support both compilers we
> >>>>>>>> must use -Wl, to forward it to the linker as is required for most other
> >>>>>>>> linker options.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Jessica Clarke <jrtc27 at jrtc27.com>
> >>>>>>>> ---
> >>>>>>>> Makefile | 57 +++++++++++++++++++++++++++++++++++++++++++++++++------
> >>>>>>>> README.md | 39 +++++++++++++++++++++++++++++++++++--
> >>>>>>>> 2 files changed, 88 insertions(+), 8 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/Makefile b/Makefile
> >>>>>>>> index 6b64205..3fe8153 100644
> >>>>>>>> --- a/Makefile
> >>>>>>>> +++ b/Makefile
> >>>>>>>> @@ -76,26 +76,54 @@ OPENSBI_VERSION_MINOR=`grep "define OPENSBI_VERSION_MINOR" $(include_dir)/sbi/sb
> >>>>>>>> OPENSBI_VERSION_GIT=$(shell if [ -d $(src_dir)/.git ]; then git describe 2> /dev/null; fi)
> >>>>>>>>
> >>>>>>>> # Setup compilation commands
> >>>>>>>> +ifneq ($(LLVM),)
> >>>>>>>> +CC = clang
> >>>>>>>> +AR = llvm-ar
> >>>>>>>> +LD = ld.lld
> >>>>>>>> +OBJCOPY = llvm-objcopy
> >>>>>>>> +else
> >>>>>>>> ifdef CROSS_COMPILE
> >>>>>>>> CC = $(CROSS_COMPILE)gcc
> >>>>>>>> -CPP = $(CROSS_COMPILE)cpp
> >>>>>>>> AR = $(CROSS_COMPILE)ar
> >>>>>>>> LD = $(CROSS_COMPILE)ld
> >>>>>>>> OBJCOPY = $(CROSS_COMPILE)objcopy
> >>>>>>>> else
> >>>>>>>> CC ?= gcc
> >>>>>>>> -CPP ?= cpp
> >>>>>>>> AR ?= ar
> >>>>>>>> LD ?= ld
> >>>>>>>> OBJCOPY ?= objcopy
> >>>>>>>> endif
> >>>>>>>> +endif
> >>>>>>>> +CPP = $(CC) -E
> >>>>>>>> AS = $(CC)
> >>>>>>>> DTC = dtc
> >>>>>>>>
> >>>>>>>> -# Guess the compillers xlen
> >>>>>>>> -OPENSBI_CC_XLEN := $(shell TMP=`$(CC) -dumpmachine | sed 's/riscv\([0-9][0-9]\).*/\1/'`; echo $${TMP})
> >>>>>>>> +ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
> >>>>>>>> +CC_IS_CLANG = y
> >>>>>>>> +else
> >>>>>>>> +CC_IS_CLANG = n
> >>>>>>>> +endif
> >>>>>>>> +
> >>>>>>>> +ifneq ($(shell $(LD) --version 2>&1 | head -n 1 | grep LLD),)
> >>>>>>>> +LD_IS_LLD = y
> >>>>>>>> +else
> >>>>>>>> +LD_IS_LLD = n
> >>>>>>>> +endif
> >>>>>>>> +
> >>>>>>>> +ifeq ($(CC_IS_CLANG),y)
> >>>>>>>> +ifneq ($(CROSS_COMPILE),)
> >>>>>>>> +CLANG_TARGET = -target $(notdir $(CROSS_COMPILE:%-=%))
> >>>>>>>
> >>>>>>> It's odd that when we use full LLVM toolchains we still need to
> >>>>>>> specify CROSS_COMPILE in order to only guess the "--target" value.
> >>>>>>> This can be written directly to --target=riscv64 / riscv32 depending
> >>>>>>> OPENSBI_CC_XLEN
> >>>>>>
> >>>>>> Hm, that’s true. To be honest, the fact that OpenSBI defaults to an
> >>>>>> unprefixed GCC is rather dubious, that’ll likely be for either
> >>>>>> riscv64-linux-gnu or riscv64-unknown-freebsd and thus not suitable
> >>>>>> (there can be subtle issues with using the wrong one, though RISC-V is
> >>>>>> more uniform than some other architectures) so should probably grab
> >>>>>> XLEN from the default GCC and then look for riscvXLEN-unknown-elf-gcc
> >>>>>> if CROSS_COMPILE wasn’t specified. I can at least make it do something
> >>>>>> like that for Clang (keep this code for CROSS_COMPILE not empty, then
> >>>>>> add a -target riscvXLEN-unknown-elf after guessing the XLEN if
> >>>>>> CROSS_COMPILE was empty).
> >>>>>
> >>>>> I don't think we should over-complicate things. Passing riscv64 /
> >>>>> riscv32 to --target is enough for OpenSBI when building with clang.
> >>>>
> >>>> We should use the right triple though, and doing so requires knowing XLEN.
> >>>>
> >>>>>>>> +endif
> >>>>>>>> +endif
> >>>>>>>> +
> >>>>>>>> +# Guess the compiler's XLEN
> >>>>>>>> +OPENSBI_CC_XLEN := $(shell TMP=`$(CC) $(CLANG_TARGET) -dumpmachine | sed 's/riscv\([0-9][0-9]\).*/\1/'`; echo $${TMP})
> >>>>>>>> +
> >>>>>>>> +# Guess the compiler's ABI and ISA
> >>>>>>>> +ifneq ($(CC_IS_CLANG),y)
> >>>>>>>> OPENSBI_CC_ABI := $(shell TMP=`$(CC) -v 2>&1 | sed -n 's/.*\(with\-abi=\([a-zA-Z0-9]*\)\).*/\2/p'`; echo $${TMP})
> >>>>>>>> OPENSBI_CC_ISA := $(shell TMP=`$(CC) -v 2>&1 | sed -n 's/.*\(with\-arch=\([a-zA-Z0-9]*\)\).*/\2/p'`; echo $${TMP})
> >>>>>>>> +endif
> >>>>>>>>
> >>>>>>>> # Setup platform XLEN
> >>>>>>>> ifndef PLATFORM_RISCV_XLEN
> >>>>>>>> @@ -194,7 +222,11 @@ else
> >>>>>>>> endif
> >>>>>>>>
> >>>>>>>> # Setup compilation commands flags
> >>>>>>>> -GENFLAGS = -I$(platform_src_dir)/include
> >>>>>>>> +ifeq ($(CC_IS_CLANG),y)
> >>>>>>>> +GENFLAGS += $(CLANG_TARGET)
> >>>>>>>> +GENFLAGS += -Wno-unused-command-line-argument
> >>>>>>>> +endif
> >>>>>>>> +GENFLAGS += -I$(platform_src_dir)/include
> >>>>>>>> GENFLAGS += -I$(include_dir)
> >>>>>>>> ifneq ($(OPENSBI_VERSION_GIT),)
> >>>>>>>> GENFLAGS += -DOPENSBI_VERSION_GIT="\"$(OPENSBI_VERSION_GIT)\""
> >>>>>>>> @@ -208,6 +240,9 @@ CFLAGS += -fno-omit-frame-pointer -fno-optimize-sibling-calls
> >>>>>>>> CFLAGS += -mno-save-restore -mstrict-align
> >>>>>>>> CFLAGS += -mabi=$(PLATFORM_RISCV_ABI) -march=$(PLATFORM_RISCV_ISA)
> >>>>>>>> CFLAGS += -mcmodel=$(PLATFORM_RISCV_CODE_MODEL)
> >>>>>>>> +ifeq ($(LD_IS_LLD),y)
> >>>>>>>> +CFLAGS += -mno-relax
> >>>>>>>> +endif
> >>>>>>>> CFLAGS += $(GENFLAGS)
> >>>>>>>> CFLAGS += $(platform-cflags-y)
> >>>>>>>> CFLAGS += -fno-pie -no-pie
> >>>>>>>> @@ -222,18 +257,28 @@ ASFLAGS += -fno-omit-frame-pointer -fno-optimize-sibling-calls
> >>>>>>>> ASFLAGS += -mno-save-restore -mstrict-align
> >>>>>>>> ASFLAGS += -mabi=$(PLATFORM_RISCV_ABI) -march=$(PLATFORM_RISCV_ISA)
> >>>>>>>> ASFLAGS += -mcmodel=$(PLATFORM_RISCV_CODE_MODEL)
> >>>>>>>> +ifeq ($(LD_IS_LLD),y)
> >>>>>>>> +ASFLAGS += -mno-relax
> >>>>>>>> +endif
> >>>>>>>> ASFLAGS += $(GENFLAGS)
> >>>>>>>> ASFLAGS += $(platform-asflags-y)
> >>>>>>>> ASFLAGS += $(firmware-asflags-y)
> >>>>>>>>
> >>>>>>>> ARFLAGS = rcs
> >>>>>>>>
> >>>>>>>> -ELFFLAGS += -Wl,--build-id=none -N -static-libgcc -lgcc
> >>>>>>>> +ifeq ($(LD_IS_LLD),y)
> >>>>>>>> +ELFFLAGS += -fuse-ld=lld
> >>>>>>>> +endif
> >>>>>>>> +ELFFLAGS += -Wl,--build-id=none -Wl,-N -static-libgcc -lgcc
> >>>>>>>> ELFFLAGS += $(platform-ldflags-y)
> >>>>>>>> ELFFLAGS += $(firmware-ldflags-y)
> >>>>>>>>
> >>>>>>>> MERGEFLAGS += -r
> >>>>>>>> +ifeq ($(LD_IS_LLD),y)
> >>>>>>>> +MERGEFLAGS += -b elf
> >>>>>>>> +else
> >>>>>>>> MERGEFLAGS += -b elf$(PLATFORM_RISCV_XLEN)-littleriscv
> >>>>>>>> +endif
> >>>>>>>> MERGEFLAGS += -m elf$(PLATFORM_RISCV_XLEN)lriscv
> >>>>>>>>
> >>>>>>>> DTSCPPFLAGS = $(CPPFLAGS) -nostdinc -nostdlib -fno-builtin -D__DTS__ -x assembler-with-cpp
> >>>>>>>> diff --git a/README.md b/README.md
> >>>>>>>> index 03c02fb..e97dcc4 100644
> >>>>>>>> --- a/README.md
> >>>>>>>> +++ b/README.md
> >>>>>>>> @@ -96,8 +96,13 @@ Required Toolchain
> >>>>>>>> ------------------
> >>>>>>>>
> >>>>>>>> OpenSBI can be compiled natively or cross-compiled on a x86 host. For
> >>>>>>>> -cross-compilation, you can build your own toolchain or just download
> >>>>>>>> -a prebuilt one from the [Bootlin toolchain repository].
> >>>>>>>> +cross-compilation, you can build your own toolchain, download a prebuilt one
> >>>>>>>> +from the [Bootlin toolchain repository] or install a distribution-provided
> >>>>>>>> +toolchain; if you opt to use LLVM/Clang, most distribution toolchains will
> >>>>>>>> +support cross-compiling for RISC-V using the same toolchain as your native
> >>>>>>>> +LLVM/Clang toolchain due to LLVM's ability to support multiple backends in the
> >>>>>>>> +same binary, so is often an easy way to obtain a working cross-compilation
> >>>>>>>> +toolchain.
> >>>>>>>>
> >>>>>>>> Please note that only a 64-bit version of the toolchain is available in
> >>>>>>>> the Bootlin toolchain repository for now.
> >>>>>>>> @@ -202,6 +207,36 @@ export PLATFORM_RISCV_XLEN=32
> >>>>>>>>
> >>>>>>>> will generate 32-bit OpenSBI images. And vice vesa.
> >>>>>>>>
> >>>>>>>> +Building with Clang/LLVM
> >>>>>>>> +------------------------
> >>>>>>>> +
> >>>>>>>> +OpenSBI can also be built with Clang/LLVM. To build with just Clang but keep
> >>>>>>>> +the default binutils (which will still use the *CROSS_COMPILE* prefix if
> >>>>>>>> +defined), override the *CC* make variable with:
> >>>>>>>> +```
> >>>>>>>> +make CC=clang
> >>>>>>>
> >>>>>>> Testing with the pre-built official LLVM 12 release for Ubuntu 20.04
> >>>>>>> [1], along with bootlin pre-built cross-compile GCC [2]:
> >>>>>>>
> >>>>>>> There are 3 build warnings when using the default binutils:
> >>>>>>>
> >>>>>>> AS platform/generic/firmware/fw_dynamic.o
> >>>>>>> firmware/fw_base.S:557:2: warning: fw_platform_init changed binding to STB_WEAK
> >>>>>>> .weak fw_platform_init
> >>>>>>> ^
> >>>>>>>
> >>>>>>> AS platform/generic/firmware/fw_jump.o
> >>>>>>> firmware/fw_base.S:557:2: warning: fw_platform_init changed binding to STB_WEAK
> >>>>>>> .weak fw_platform_init
> >>>>>>> ^
> >>>>>>>
> >>>>>>> AS platform/generic/firmware/fw_payload.o
> >>>>>>> firmware/fw_base.S:557:2: warning: fw_platform_init changed binding to STB_WEAK
> >>>>>>> .weak fw_platform_init
> >>>>>>> ^
> >>>>>>
> >>>>>> Yeah, those are known. They’re harmless and easy to fix (just delete
> >>>>>> the .globl lines, as the two are mutually exclusive), so I didn’t
> >>>>>> include it as part of this patch series, LLVM 12 just got stricter
> >>>>>> about this as it’s dodgy code.
> >>>>>
> >>>>> Please include a patch to fix these warnings as part of this series.
> >>>>> We should not allow any building warnings to happen.
> >>>>
> >>>> Ok, that’s a trivial patch to include in v3.
> >>>>
> >>>>>>> And several warnings from the GNU linker:
> >>>>>>>
> >>>>>>> /share/toolchains/riscv64/bin/riscv64-linux-ld: warning: library
> >>>>>>> search path "/lib/../lib64" is unsafe for cross-compilation
> >>>>>>> /share/toolchains/riscv64/bin/riscv64-linux-ld: warning: library
> >>>>>>> search path "/usr/lib/../lib64" is unsafe for cross-compilation
> >>>>>>> /share/toolchains/riscv64/bin/riscv64-linux-ld: warning: library
> >>>>>>> search path "/lib" is unsafe for cross-compilation
> >>>>>>> /share/toolchains/riscv64/bin/riscv64-linux-ld: warning: library
> >>>>>>> search path "/usr/lib" is unsafe for cross-compilation
> >>>>>>
> >>>>>> Hm, indeed, Clang likes to add some host directories to the search
> >>>>>> path, seemingly only for RISC-V. RISC-V’s bare-metal toolchain driver
> >>>>>> is a bit quirky due to all the multilib stuff the GNU world decided to
> >>>>>> introduce so that’s a bug. They should be harmless though given we
> >>>>>> don’t pass -lfoo, when linking in SBI libraries we pass the path.
> >>>>>
> >>>>> Is it a bug of clang, or GNU ld? Could you please file a bug report to
> >>>>> get this issue tracked (if there isn't one already), and mentioned in
> >>>>> the commit message?
> >>>>
> >>>> Clang, though I suspect some of these are a result of you using the
> >>>> wrong triple (see below).
> >>>>
> >>>>>>> The generated fw_dynamic firmware image does not boot on QEMU 'virt'.
> >>>>>>> Initial debugging shows that it returns SBI_EINVAL in
> >>>>>>> sanitize_domain().
> >>>>>>
> >>>>>> My pure LLVM=1-built fw_dynamic binary on FreeBSD boots U-Boot just fine
> >>>>>> (-M virt -m 2048 -nographic -bios fw_dynamic.elf -kernel u-boot), as
> >>>>>> does an LLVM=1 with LD overridden to be GNU ld 2.33.1 (also on FreeBSD)
> >>>>>> so I don’t know what’s going on there. Though I did discover that
> >>>>>> -fuse-ld=bfd is needed for the non-LLD case otherwise Clang won’t pick
> >>>>>> up an ld.bfd intended to override a system LLD. So my guess is that one
> >>>>>> or other of the binaries you downloaded has broken something. Does
> >>>>>> using LLD work? If you tell me exactly what you ran I can try it on a
> >>>>>> Linux machine myself and see if I can reproduce it.
> >>>>>
> >>>>> Switching to full LLVM build does not build for me.
> >>>>>
> >>>>> ELF platform/generic/firmware/fw_dynamic.elf
> >>>>> ld.lld: error: can't create dynamic relocation R_RISCV_64 against
> >>>>> symbol: _fw_start in readonly segment; recompile object files with
> >>>>> -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output
> >>>>>>>> defined in opensbi/build/platform/generic/firmware/fw_dynamic.elf.ld:8
> >>>>>>>> referenced by fw_base.S:502 (opensbi/firmware/fw_base.S:502)
> >>>>>>>> opensbi/build/platform/generic/firmware/fw_dynamic.o:(.entry+0x3A0)
> >>>>>
> >>>>> ld.lld: error: can't create dynamic relocation R_RISCV_64 against
> >>>>> symbol: _fw_reloc_end in readonly segment; recompile object files with
> >>>>> -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output
> >>>>>>>> defined in opensbi/build/platform/generic/firmware/fw_dynamic.elf.ld:92
> >>>>>>>> referenced by fw_base.S:502 (opensbi/firmware/fw_base.S:502)
> >>>>>>>> opensbi/build/platform/generic/firmware/fw_dynamic.o:(.entry+0x3B0)
> >>>>> clang-12: error: linker command failed with exit code 1 (use -v to see
> >>>>> invocation)
> >>>>> make: *** [Makefile:396:
> >>>>> opensbi/build/platform/generic/firmware/fw_dynamic.elf] Error 1
> >>>>
> >>>> This is a result of you using the wrong triple, though it’s also a sign
> >>>> that we do slightly bogus things. For FW_PIC we link with -pie, but
> >>>> that doesn’t make sense for bare-metal. We also don’t link with
> >>>> -static, but it’s the default (and only) supported thing for bare-metal
> >>>> targets. If you use a bare-metal triple then the -pie gets ignored (we
> >>>> should probably remove it from objects.mk) and -static is implied. If
> >>>> you use a Linux triple then the -pie gets honoured and the lack of
> >>>> -static means it defaults to dynamic linking, so LLD rightly complains
> >>>> about the fact that fw_base.S is creating a pointer in a read-only
> >>>> section that requires run-time relocation. I don’t know why you don’t
> >>>> see the same thing with GNU ld but it’s probably just silently allowing
> >>>> it and leaving it to crash at run time.
> >>>
> >>> I am confused. Did you mean "riscv64-linux-" is a bare-metal triple? I
> >>> thought "riscv64-unknown-elf-" is one bare-metal triple, and "-target
> >>> riscv64" is too.
> >>>
> >>> I changed to pass "-target riscv64" to clang, and now it builds and
> >>> boots fine with LLVM=1 case.
> >>
> >> I further looked at this one. Even passing "-target riscv64" leads to
> >> a successful build and boot to U-Boot, I checked the generated ELF
> >> image and found the .rela.dyn section is empty.
> >
> > By hardcoding "-target riscv64-unknown-elf", and
> >
> > $ make LLVM=1 PLATFORM=generic
> >
> > I got the same result as "-target riscv64". It builds and boots, but
> > with an empty.rela.dyn
>
> If unspecified, the vendor and OS default to unknown and elf
> respectively, so the two are entirely equivalent, but I felt it best to
> be explicit, especially since anyone without an LLVM background reading
> the Makefile might be confused.
>
Ah, thanks! I am not aware of this clang triple convention :)
> This is a bare-metal binary, of course .rela.dyn is going to be empty,
> there’s no run-time linker to do any relocations. How on earth is the
> FW_PIC support supposed to work?
See commit 0f20e8adcf42 ("firmware: Support position independent
execution") for the FW_PIC changes. Basically OpenSBI relocates itself
at the very beginning of the boot phase if building with -fpie.
> That looks utterly broken to me. Is it *intended* to be abusing riscv64-linux-gnu? Because that’s just plain wrong...
I am not sure what you mean by "abusing riscv64-linux-gnu"? But this
-fpie stuff is perfectly okay and commonly used by every architecture
in the U-Boot world.
Regards,
Bin
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