Hi Koakuma, On Thu, Jun 20, 2024 at 10:56:00PM +0700, Koakuma via B4 Relay wrote: > From: Koakuma <koachan@xxxxxxxxxxxxxx> > > Remove flags not supported by clang and make sure that all the flags > used are portable between clang and GCC. > > The reasoning for removing the -fcall-used* ones is as follows: > > In the (normal) 32-bit ABI, %g5 and %g7 is normally reserved, and in > the 64-bit ABI, %g7 is the reserved one. > Linux turns them into volatile registers by the way of -fcall-used-*, > but on the other hand, omitting the flags shouldn't be harmful; > compilers will now simply refuse to touch them, and any assembly > code that happens to touch them would still work like usual (because > Linux' conventions already treats them as volatile anyway). > > Signed-off-by: Koakuma <koachan@xxxxxxxxxxxxxx> > --- > Hello~ > > This changes the CFLAGS for building the SPARC kernel so that it can be > built with clang, as a follow up from the discussion in this thread: > > https://lore.kernel.org/lkml/JAYB7uS-EdLABTR4iWZdtFOVa5MvlKosIrD_cKTzgeozCOGRM7lhxeLigFB1g3exX445I_W5VKB-tAzl2_G1zCVJRQjp67ODfsSqiZWOZ9o=@protonmail.com/T/#u > > The changes are removal of various `-fcall-used-*` flags, and changing > `-mv8plus` to `-mcpu=v9`: > > - `-fcall-used-*` flags should be safe to remove; the compiler will > stop using the registers specified as temporaries, but it is a safe > change wrt. the ABI. Assembly code can still use those registers > as needed. > It does bring a theoretical possible slowdown due to the compiler > having less registers to work with, but in practice - in my case, > at least - it seems to not make any difference with daily usage. > > - More trivial is to change `-mv8plus` -> `-mcpu=v9`. > This should be safe too since the kernel seems to require a V9 > processor to run anyway, so I'm changing the flag to one that is > portable between GCC and clang. > > Also, as stated in the thread, building with these changes still result > in a working kernel, at least for Sun T5120 and qemu virtual machines. > > On the LLVM side, the effort for building Linux/SPARC is tracked here: > https://github.com/llvm/llvm-project/issues/40792 This is really awesome to see, thanks for sending this patch! I think a good amount of the reasoning below the '---' could probably make it into the commit message as well but I don't have much of a vision there, maybe one of the SPARC folks will. I saw through the LLVM issue above that one other patch is necessary to fix an issue in the vDSO [1], which I applied in testing this one. I noticed in applying that change that you appear to be working on 6.1, which is fine for now, but you'll need another diff once you get to a newer version, as we stopped using CROSS_COMPILE to set clang's '--target=' value: diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang index 6c23c6af797f..2435efae67f5 100644 --- a/scripts/Makefile.clang +++ b/scripts/Makefile.clang @@ -10,6 +10,7 @@ CLANG_TARGET_FLAGS_mips := mipsel-linux-gnu CLANG_TARGET_FLAGS_powerpc := powerpc64le-linux-gnu CLANG_TARGET_FLAGS_riscv := riscv64-linux-gnu CLANG_TARGET_FLAGS_s390 := s390x-linux-gnu +CLANG_TARGET_FLAGS_sparc := sparc64-linux-gnu CLANG_TARGET_FLAGS_x86 := x86_64-linux-gnu CLANG_TARGET_FLAGS_um := $(CLANG_TARGET_FLAGS_$(SUBARCH)) CLANG_TARGET_FLAGS := $(CLANG_TARGET_FLAGS_$(SRCARCH)) With those, I can successfully build a kernel with clang that boots in QEMU :) $ make -skj"$(nproc)" \ ARCH=sparc64 \ CC=clang \ CROSS_COMPILE=sparc64-linux-gnu- \ LLVM_IAS=0 \ mrproper defconfig all $ qemu-system-sparc64 \ -serial mon:stdio \ -display none \ -no-reboot \ -M sun4u \ -cpu 'TI UltraSparc IIi' \ -m 512 \ -append console=ttyS0 \ -initrd sparc64-rootfs.cpio \ -kernel arch/sparc/boot/image ... [ 1.788544] Run /init as init process ... Linux version 6.10.0-rc4+ (nathan@thelio-3990X) (ClangBuiltLinux clang version 19.0.0git (https://github.com/llvm/llvm-project a083e50f53f0f9eb9ad0c5b65f3c627cf97043e6), GNU ld (GNU Binutils) 2.42) #1 SMP Fri Jun 21 11:36:18 MST 2024 ... Consider this: Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx> [1]: https://github.com/koachan/linux-clang/commit/c0114bfc7a4f64bc4d3e63eca6582ec827a8e2a2 > --- > arch/sparc/Makefile | 4 ++-- > arch/sparc/vdso/Makefile | 4 ++-- > 2 files changed, 4 insertions(+), 4 deletions(-) > > diff --git a/arch/sparc/Makefile b/arch/sparc/Makefile > index 757451c3ea1d..7318a8b452c3 100644 > --- a/arch/sparc/Makefile > +++ b/arch/sparc/Makefile > @@ -29,7 +29,7 @@ UTS_MACHINE := sparc > # versions of gcc. Some gcc versions won't pass -Av8 to binutils when you > # give -mcpu=v8. This silently worked with older bintutils versions but > # does not any more. > -KBUILD_CFLAGS += -m32 -mcpu=v8 -pipe -mno-fpu -fcall-used-g5 -fcall-used-g7 > +KBUILD_CFLAGS += -m32 -mcpu=v8 -pipe -mno-fpu > KBUILD_CFLAGS += -Wa,-Av8 > > KBUILD_AFLAGS += -m32 -Wa,-Av8 > @@ -45,7 +45,7 @@ export BITS := 64 > UTS_MACHINE := sparc64 > > KBUILD_CFLAGS += -m64 -pipe -mno-fpu -mcpu=ultrasparc -mcmodel=medlow > -KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 -fcall-used-g7 -Wno-sign-compare > +KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 -Wno-sign-compare > KBUILD_CFLAGS += -Wa,--undeclared-regs > KBUILD_CFLAGS += $(call cc-option,-mtune=ultrasparc3) > KBUILD_AFLAGS += -m64 -mcpu=ultrasparc -Wa,--undeclared-regs > diff --git a/arch/sparc/vdso/Makefile b/arch/sparc/vdso/Makefile > index 243dbfc4609d..929140facabf 100644 > --- a/arch/sparc/vdso/Makefile > +++ b/arch/sparc/vdso/Makefile > @@ -46,7 +46,7 @@ CFL := $(PROFILING) -mcmodel=medlow -fPIC -O2 -fasynchronous-unwind-tables -m64 > -fno-omit-frame-pointer -foptimize-sibling-calls \ > -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO > > -SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5 -fcall-used-g5 -fcall-used-g7 > +SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5 > > $(vobjs): KBUILD_CFLAGS := $(filter-out $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(SPARC_REG_CFLAGS),$(KBUILD_CFLAGS)) $(CFL) > > @@ -86,7 +86,7 @@ KBUILD_CFLAGS_32 += -fno-stack-protector > KBUILD_CFLAGS_32 += $(call cc-option, -foptimize-sibling-calls) > KBUILD_CFLAGS_32 += -fno-omit-frame-pointer > KBUILD_CFLAGS_32 += -DDISABLE_BRANCH_PROFILING > -KBUILD_CFLAGS_32 += -mv8plus > +KBUILD_CFLAGS_32 += -mcpu=v9 > $(obj)/vdso32.so.dbg: KBUILD_CFLAGS = $(KBUILD_CFLAGS_32) > > $(obj)/vdso32.so.dbg: FORCE \ > > --- > base-commit: 92e5605a199efbaee59fb19e15d6cc2103a04ec2 > change-id: 20240620-sparc-cflags-e7f2dbbd4b9d > > Best regards, > -- > Koakuma <koachan@xxxxxxxxxxxxxx> > >
