Messages in this thread | | | Date | Wed, 12 Feb 2020 11:35:38 -0700 | From | Nathan Chancellor <> | Subject | Re: Status of building and booting the RISCV64 kernel with Clang |
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On Wed, Feb 12, 2020 at 10:29:10AM -0800, 'Nick Desaulniers' via Clang Built Linux wrote: > + Palmer, Alex > > On Tue, Feb 11, 2020 at 9:40 PM Nathan Chancellor > <natechancellor@gmail.com> wrote: > > > > Hi all, > > > > We (ClangBuiltLinux) have started looking at building the RISCV64 > > kernel with clang and booting it in QEMU. I thought it would be nice to > > give some sort of status update and go over the issues we have notice > > so far and see what people's thoughts are for solving them. If this > > email is unwelcome, apologies! This testing was done with clang-11 (tip > > of tree) and QEMU 4.2.0 with this rootfs (built with > > buildroot-2019.02.9): > > > > https://github.com/nathanchance/continuous-integration/blob/21ea056022f9cf1d62b7f805c3862daf9d89736f/images/riscv/rootfs.cpio > > > > > > > > Outstanding issues on the LLVM side: > > > > 1. LLVM output over -mno-save-restore (https://github.com/ClangBuiltLinux/linux/issues/804) > > > > The kernel passes -mno-save-restore to KBUILD_CFLAGS which causes a ton > > of warning spam from LLVM: > > > > '-save-restore' is not a recognized feature for this target (ignoring feature) > > > > This has been reported upstream as https://llvm.org/pr44853 but it could > > easily be fixed in the kernel by guarding the call against > > CONFIG_CC_IS_CLANG (although this is obviously fragile if the ABI ever > > changes to -msave-restore by default).
And this issue has been fixed: https://github.com/llvm/llvm-project/commit/07f7c00208b393296f8f27d6cd3cec2b11d86fd8
> > 2. -fPIC causes issues when using Clang + GNU as (https://github.com/ClangBuiltLinux/linux/issues/865) > > > > The kernel on a whole builds with -fno-integrated-as so we fall back to > > GNU as when assembling. Kernel modules are built with -fPIC and fail to > > build with a bunch of assembler errors like so (seen with binutils > > 2.31.1 and ToT): > > > > /tmp/flexfilelayout-2d0cdc.s:359: Error: bad expression > > /tmp/flexfilelayout-2d0cdc.s:359: Error: illegal operands `auipc a0,%got_pcrel_hi(mem_map)' > > /tmp/flexfilelayout-2d0cdc.s:367: Error: bad expression > > /tmp/flexfilelayout-2d0cdc.s:367: Error: illegal operands `auipc a2,%got_pcrel_hi(pfn_base)' > > /tmp/flexfilelayout-2d0cdc.s:374: Error: bad expression > > /tmp/flexfilelayout-2d0cdc.s:374: Error: illegal operands `auipc a3,%got_pcrel_hi(va_pa_offset)' > > > > This has been reported upstream as https://llvm.org/pr44854. The way to > > work around this in the kernel is to just disable CONFIG_MODULES (which > > is a big hammer but that obviously won't be sent upstream in any form). > > > > > > > > > > Outstanding issues on the kernel side: > > > > 1. -Wuninitialized warnings around local register variables > > > > There are a few warnings around local "register" variables, which are > > uninitialized when using clang: > > > > In file included from ../arch/riscv/kernel/asm-offsets.c:10: > > In file included from ../include/linux/sched.h:12: > > ../arch/riscv/include/asm/current.h:30:9: warning: variable 'tp' is uninitialized when used here [-Wuninitialized] > > return tp; > > ^~ > > ../arch/riscv/include/asm/current.h:29:33: note: initialize the variable 'tp' to silence this warning > > register struct task_struct *tp __asm__("tp"); > > ^ > > = NULL > > 1 warning generated. > > > > ../arch/riscv/kernel/process.c:112:19: warning: variable 'gp' is uninitialized when used here [-Wuninitialized] > > childregs->gp = gp; > > ^~ > > ../arch/riscv/kernel/process.c:110:34: note: initialize the variable 'gp' to silence this warning > > const register unsigned long gp __asm__ ("gp"); > > ^ > > = 0 > > 1 warning generated. > > > > ../arch/riscv/kernel/stacktrace.c:34:8: warning: variable 'current_sp' is uninitialized when used here [-Wuninitialized] > > sp = current_sp; > > ^~~~~~~~~~ > > ../arch/riscv/kernel/stacktrace.c:32:42: note: initialize the variable 'current_sp' to silence this warning > > const register unsigned long current_sp __asm__ ("sp"); > > ^ > > = 0 > > 1 warning generated. > > > > The way to solve these is to make these register variables global, where > > they are properly initialized and work. This has been done in the kernel > > a few times: > > > > fe92da0f355e ("MIPS: Changed current_thread_info() to an equivalent supported by both clang and GCC") > > 3337a10e0d0c ("arm64: LLVMLinux: Add current_stack_pointer() for arm64") > > 786248705ecf ("arm64: LLVMLinux: Calculate current_thread_info from current_stack_pointer") > > 0abc08baf2dd ("ARM: 8170/1: Add global named register current_stack_pointer for ARM") > > f6c9cbf091a4 ("ARM: 8173/1: Calculate current_thread_info from current_stack_pointer") > > > > The LLVM community has rejected adopting GCC's behavior of allowing > > local register variables because it would seriously complicate the > > register allocator; the full discussion can be viewed here: > > http://lists.llvm.org/pipermail/llvm-dev/2014-March/071472.html > > > > This is the diff I am currently working with; I am not sure of any side > > effects aside from two that I will list below. > > > > https://gist.github.com/b5fda253a243127736fd2ac5d317dcdd > > > > > > > > Booting in QEMU: > > > > This is where things get interesting... The kernel does not start at all > > when the registers are purely local. It does start when the tp register > > is moved globally (arch/riscv/include/asm/current.h diff above) but it > > does not finish getting to userspace. Additionally, the diff in > > -s ARCH=riscv CC=clang CROSS_COMPILE=riscv64-linux-gnu- O=out.riscv distclean defconfig all > > ... > > $ timeout 30s qemu-system-riscv64 -M virt -m 512M -no-reboot -bios default -kernel out.riscv/arch/riscv/boot/Image -display none -serial mon:stdio -initrd out.riscv/rootfs.cpio > > ... > > [ 0.000000] Linux version 5.6.0-rc1-00001-g90c81dfc010e (nathan@ubuntu-m2-xlarge-x86) (ClangBuiltLinux clang version 11.0.0 (git://github.com/llvm/llvm-project 9c1a88c96457ffde71f13c74fd4d52a77d86cc9f)) #1 SMP Tue Feb 11 22:13:03 MST 2020 > > ... > > [ 0.624295] Run /init as init process > > /init: exec: line 7: /sbin/init: Text file busy > > [ 0.712090] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000200 > > [ 0.712739] CPU: 0 PID: 1 Comm: init Not tainted 5.6.0-rc1-00001-g90c81dfc010e #1 > > [ 0.713228] Call Trace: > > [ 0.713508] [<ffffffe00004a3e6>] walk_stackframe+0x0/0xc6 > > [ 0.713832] [<ffffffe0007c0070>] dump_stack+0x9e/0xd6 > > [ 0.714112] [<ffffffe00004f250>] panic+0x112/0x2dc > > [ 0.714387] [<ffffffe000051886>] exit_mm+0x0/0x12a > > [ 0.714676] [<ffffffe000051a80>] sys_exit_group+0x0/0xe > > [ 0.714965] [<ffffffe000051aa4>] __wake_up_parent+0x0/0x24 > > [ 0.715262] [<ffffffe000051a8e>] __do_sys_exit_group+0x0/0x16 > > [ 0.715568] [<ffffffe000048e3e>] ret_from_syscall+0x0/0x2 > > [ 0.716409] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000200 ]--- > > qemu-system-riscv64: terminating on signal 15 from pid 55965 (timeout) > > > > GCC 9.2.0 works just fine. > > > > $ make -j$(nproc) -s ARCH=riscv CROSS_COMPILE=riscv64-linux- O=out.riscv distclean defconfig all > > $ timeout 30s qemu-system-riscv64 -M virt -m 512M -no-reboot -bios default -kernel out.riscv/arch/riscv/boot/Image -display none -serial mon:stdio -initrd out.riscv/rootfs.cpio > > ... > > [ 0.634854] Run /init as init process > > Starting syslogd: OK > > Starting klogd: OK > > Initializing random number generator... [ 1.329410] random: dd: uninitialized urandom read (512 bytes read) > > done. > > Starting network: OK > > Linux version 5.6.0-rc1-00001-g90c81dfc010e (nathan@ubuntu-m2-xlarge-x86) (gcc version 9.2.0 (GCC)) #1 SMP Tue Feb 11 22:20:36 MST 2020 > > Linux version 5.6.0-rc1-00001-g90c81dfc010e (nathan@ubuntu-m2-xlarge-x86) (gcc version 9.2.0 (GCC)) #1 SMP Tue Feb 11 22:20:36 MST 2020 > > Stopping network: OK > > Saving random seed... [ 2.165960] random: dd: uninitialized urandom read (512 bytes read) > > done. > > Stopping klogd: OK > > Stopping syslogd: OK > > umount: devtmpfs busy - remounted read-only > > umount: can't unmount /: Invalid argument > > The system is going down NOW! > > Sent SIGTERM to all processes > > Sent SIGKILL to all processes > > Requesting system poweroff > > [ 4.412388] reboot: Power down > > > > I have tried to do some debugging in gdb to see where things are going > > wrong and I see it get to run_init_process, succeed, then jump to the > > exception handler and panic so I am not really sure where things are > > going wrong. Any sort of ideas on where to go from here would certainly > > be appreciated :) > > > > Thanks for all the hard work everyone has done, hopefully we can help > > add to it! > > > > Cheers, > > Nathan
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