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SubjectRe: Status of building and booting the RISCV64 kernel with Clang
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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