linux-kernel - Re: [PATCH v6] arm64: implement ftrace with regs

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Message-ID: <6ac63f70-d9a4-6b56-ed59-fc18873d8293@arm.com>
Date:   Wed, 16 Jan 2019 10:08:38 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Torsten Duwe <duwe@....de>, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v6] arm64: implement ftrace with regs



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@...e.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