Re: [PATCH v6] arm64: implement ftrace with regs

[Julien Thierry <julien.thierry@arm.com>]

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?

Thanks,

-- 
Julien Thierry