* Function Tracing with clang on ARM32
@ 2018-08-11 12:29 Stefan Agner
2018-08-24 21:20 ` Matthias Kaehlcke
0 siblings, 1 reply; 3+ messages in thread
From: Stefan Agner @ 2018-08-11 12:29 UTC (permalink / raw)
To: linux-arm-kernel
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
^ permalink raw reply [flat|nested] 3+ messages in thread* Function Tracing with clang on ARM32 2018-08-11 12:29 Function Tracing with clang on ARM32 Stefan Agner @ 2018-08-24 21:20 ` Matthias Kaehlcke 2018-08-26 19:22 ` Stefan Agner 0 siblings, 1 reply; 3+ messages in thread From: Matthias Kaehlcke @ 2018-08-24 21:20 UTC (permalink / raw) To: linux-arm-kernel Hi Stefan, A similar/related issue was fixed for arm64 by https://reviews.llvm.org/rL266039 The corresponding LLVM bug is https://bugs.llvm.org//show_bug.cgi?id=27311 Apparently for arm32 the use of '-meabi gnu' was chosen as solution: http://llvm.org/viewvc/llvm-project?view=revision&revision=265888 Cheers Matthias On Sat, Aug 11, 2018 at 02:29:24PM +0200, Stefan Agner wrote: > 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? > ^ permalink raw reply [flat|nested] 3+ messages in thread
* Function Tracing with clang on ARM32 2018-08-24 21:20 ` Matthias Kaehlcke @ 2018-08-26 19:22 ` Stefan Agner 0 siblings, 0 replies; 3+ messages in thread From: Stefan Agner @ 2018-08-26 19:22 UTC (permalink / raw) To: linux-arm-kernel On 24.08.2018 23:20, Matthias Kaehlcke wrote: > Hi Stefan, > > A similar/related issue was fixed for arm64 by > https://reviews.llvm.org/rL266039 > > The corresponding LLVM bug is > https://bugs.llvm.org//show_bug.cgi?id=27311 > > Apparently for arm32 the use of '-meabi gnu' was chosen as solution: > > http://llvm.org/viewvc/llvm-project?view=revision&revision=265888 Yes I did saw that, and using -meabi gnu allows to compile the kernel with CONFIG_FUNCTION_TRACER=y (see below). Unfortunately, when trying to boot the kernel crashes very early. I dig a bit deeper and found that the function entry looks slightly different when using clang: clang v6.0.1 with 0000026c <proc_caches_init>: 26c: b5d0 push {r4, r6, r7, lr} 26e: af02 add r7, sp, #8 270: b084 sub sp, #16 272: f7ff fffe bl 0 <__gnu_mcount_nc> GCC 7.2.1: 000001d6 <proc_caches_init>: 1d6: b530 push {r4, r5, lr} 1d8: b085 sub sp, #20 1da: b500 push {lr} 1dc: f7ff fffe bl 0 <__gnu_mcount_nc> It seems that GCC stores the link register on the stack, whereas clang stores a reference to the location of the stack where r7/lr has been stored. Changing mcount_exit to not restore lr seems to help: .macro mcount_exit ldmia sp!, {r0-r3, ip} ret ip .endm However, this leads to a broken trace, since mcount_get_lr can't find the link register... ... <idle>-0 [000] dn.. 59.055202: hrtimer_start_range_ns <-init_stack <idle>-0 [000] dn.. 59.055203: _raw_spin_lock_irqsave <-__gnu_mcount_nc <idle>-0 [000] dn.. 59.055205: hrtimer_reprogram <-0xdf946580 <idle>-0 [000] dn.. 59.055206: tick_program_event <-0x0000000d <idle>-0 [000] dn.. 59.055208: clockevents_program_event <-0xdf946860 ... It seems somewhat odd that clang uses the GCC function name but does not provide lr on the stack... -- Stefan > > Cheers > > Matthias > > On Sat, Aug 11, 2018 at 02:29:24PM +0200, Stefan Agner wrote: >> 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? >> ^ permalink raw reply [flat|nested] 3+ messages in thread
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