From: stefan@agner.ch (Stefan Agner)
To: linux-arm-kernel@lists.infradead.org
Subject: Function Tracing with clang on ARM32
Date: Sat, 11 Aug 2018 14:29:24 +0200 [thread overview]
Message-ID: <35ae3d464c7b77a5fe86a732079e32bc@agner.ch> (raw)
Hi,
I am currently looking into function tracing support when compiling an
ARM32 kernel with clang.
With v4.18-rc1 and:
CONFIG_FTRACE=y
CONFIG_FUNCTION_TRACER=y
Building fails:
LD vmlinux.o
MODPOST vmlinux.o
kernel/softirq.o: In function `_local_bh_enable':
/home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:152:
undefined reference to `mcount'
kernel/softirq.o: In function `__local_bh_enable_ip':
/home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:159:
undefined reference to `mcount'
kernel/softirq.o: In function `do_softirq':
/home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:316:
undefined reference to `mcount'
kernel/softirq.o: In function `irq_enter':
/home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:337:
undefined reference to `mcount'
kernel/softirq.o: In function `irq_exit':
/home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:395:
undefined reference to `mcount'
It seems that clang/LLVM uses a different mcount interface. However,
there is an option to use the gcc compatible symbol __gnu_mcount_nc when
using -meabi gnu. So adding
CFLAGS_ABI +=-meabi gnu
Builds the kernel successfully. However, the kernel crashes immediately
on boot. Even with earlyprintk enabled, I only get:
Uncompressing Linux... done, booting the kernel.
I poked a bit around in arch/arm/kernel/entry-ftrace.S. Not calling
__mcount in __gnu_mcount_nc produces a bootable kernel. Removing the
call into the ftrace logic by only using mcount_enter/exit in the
__mcount macro does not boot...
I guess the mcount ABI is slightly different in LLVM/clang still then?
Before I dive deeper, maybe anyone has an idea?
--
Stefan
next reply other threads:[~2018-08-11 12:29 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-11 12:29 Stefan Agner [this message]
2018-08-24 21:20 ` Function Tracing with clang on ARM32 Matthias Kaehlcke
2018-08-26 19:22 ` Stefan Agner
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