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SubjectRe: [PATCH v6] arm64: implement ftrace with regs
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On 16/01/2019 09:57, Julien Thierry wrote:
> Hi Torsten,
>
> On 04/01/2019 14:10, Torsten Duwe wrote:
>> Use -fpatchable-function-entry (gcc8) to add 2 NOPs at the beginning
>> of each function. Replace the first NOP thus generated with a quick LR
>> saver (move it to scratch reg x9), so the 2nd replacement insn, the call
>> to ftrace, does not clobber the value. Ftrace will then generate the
>> standard stack frames.
>>
>> Note that patchable-function-entry in GCC disables IPA-RA, which means
>> ABI register calling conventions are obeyed *and* scratch registers
>> such as x9 are available.
>>
>> Introduce and handle an ftrace_regs_trampoline for module PLTs, right
>> after ftrace_trampoline, and double the size of this special section.
>>
>> Signed-off-by: Torsten Duwe <duwe@suse.de>
>>
>
> I wanted to test this patch (and try to benchmark having the "mov x9,
> x30" always present in function prelude vs having two nops), but I
> cannot get this patch to apply (despite having a version including both
> commits below).
>
> Could you provide a git branch from which I could try to rebase the
> patch? (Or a new version of the series)
>
>> ---
>>
>> This patch applies on 4.20 with the additional changes
>> bdb85cd1d20669dfae813555dddb745ad09323ba
>> (arm64/module: switch to ADRP/ADD sequences for PLT entries)
>> and
>> 7dc48bf96aa0fc8aa5b38cc3e5c36ac03171e680
>> (arm64: ftrace: always pass instrumented pc in x0)
>> along with their respective series, or alternatively on Linus' master,
>> which already has these.
>>
>> changes since v5:
>>
>> * fix mentioned pc in x0 to hold the start address of the call site,
>> not the return address or the branch address.
>> This resolves the problem found by Amit.
>>
>> ---
>> arch/arm64/Kconfig | 2
>> arch/arm64/Makefile | 4 +
>> arch/arm64/include/asm/assembler.h | 1
>> arch/arm64/include/asm/ftrace.h | 13 +++
>> arch/arm64/include/asm/module.h | 3
>> arch/arm64/kernel/Makefile | 6 -
>> arch/arm64/kernel/entry-ftrace.S | 131 ++++++++++++++++++++++++++++++++++
>> arch/arm64/kernel/ftrace.c | 125 ++++++++++++++++++++++++--------
>> arch/arm64/kernel/module-plts.c | 3
>> arch/arm64/kernel/module.c | 2
>> drivers/firmware/efi/libstub/Makefile | 3
>> include/asm-generic/vmlinux.lds.h | 1
>> include/linux/compiler_types.h | 4 +
>> 13 files changed, 262 insertions(+), 36 deletions(-)
>
> [...]
>
>> --- a/arch/arm64/kernel/entry-ftrace.S
>> +++ b/arch/arm64/kernel/entry-ftrace.S
>
> [...]
>
>> @@ -122,6 +124,7 @@ skip_ftrace_call: // }
>> ENDPROC(_mcount)
>>
>> #else /* CONFIG_DYNAMIC_FTRACE */
>> +#ifndef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>> /*
>> * _mcount() is used to build the kernel with -pg option, but all the branch
>> * instructions to _mcount() are replaced to NOP initially at kernel start up,
>> @@ -159,6 +162,124 @@ GLOBAL(ftrace_graph_call) // ftrace_gra
>>
>> mcount_exit
>> ENDPROC(ftrace_caller)
>> +#else /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
>> +
>> +/*
>> + * Since no -pg or similar compiler flag is used, there should really be
>> + * no reference to _mcount; so do not define one. Only some value for
>> + * MCOUNT_ADDR is needed for comparison. Let it point here to have some
>> + * sort of magic value that can be recognised when debugging.
>> + */
>> +GLOBAL(_mcount)
>> + ret /* make it differ from regs caller */
>
> There's something I can't figure out. Since there are no callers to
> _mcount, how does the ftrace core builds up its record of patchable
> functions?
>
> I don't understand fully the core ftrace code but I've got the
> impression that without this record of struct dyn_ftrace, ftrace cannot
> patch in calls to tracers in the future.
>
> Am I missing something?
>

Forget that second part, I just saw the vmlinux.lds.h change.

--
Julien Thierry

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