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

[Nick Desaulniers <ndesaulniers@google.com>]

+ 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).
>
> 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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-- 
Thanks,
~Nick Desaulniers