Re: [PATCH] s390: add support for DYNAMIC_FTRACE_WITH_REGS

Heiko Carstens
2014-07-08T08:07:40Z

On Thu, Jul 03, 2014 at 02:00:46PM +0200, Vojtech Pavlik wrote:
> Add support for DYNAMIC_FTRACE_WITH_REGS to 64-bit and 31-bit s390
> architectures. This is required for kGraft and kpatch to work on s390.
>
> It's done by adding a _regs variant of ftrace_caller that preserves
> registers and puts them on stack in a struct pt_regs layout and
> allows modification of return address by changing the PSW (instruction
> pointer) member od struct pt_regs.
>
> Signed-off-by: Vojtech Pavlik <vojtech@suse.cz>
> Reviewed-by: Jiri Kosina <jkosina@suse.cz>
> Cc: Steven Rostedt <rostedt@goodmis.org>

So I assume you use the instruction_pointer() macro to access the
return address then?

All of this seems a bit of a hack to me.. the natural place of the
return address of a function would be register 14, and not the
psw member of the pt_regs structure.

It's then also inconsistent to only save register r0-r13 to the
gprs member.. well, you can't save r14, since what should
happen if both r14 in the gprs member of pt_regs and in the psw
part would have been changed?

Besides that a couple more comments below.

> diff --git a/arch/s390/kernel/mcount64.S b/arch/s390/kernel/mcount64.S
> index 1c52eae..bcad958 100644
> --- a/arch/s390/kernel/mcount64.S
> +++ b/arch/s390/kernel/mcount64.S
> @@ -49,6 +49,44 @@ ENTRY(ftrace_graph_caller)
> lg %r14,112(%r15)
> br %r14
>
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +ENTRY(ftrace_regs_caller)
> + larl %r1,function_trace_stop
> + icm %r1,0xf,0(%r1)
> + bnzr %r14

The three lines above should go away, but that's not your problem, since
Steven is about to remove the function_trace_stop functionality.

> + lgr %r1,%r15
> + aghi %r15,-336
> + stg %r1,__SF_BACKCHAIN(%r15)
> + stg %r1,304(%r15)
> + stmg %r0,%r13,184(%r15)
> + stg %r14,176(%r15)
> + lgr %r2,%r14
> + aghi %r2,-MCOUNT_INSN_SIZE
> + lg %r3,344(%r15)
> + stg %r3,296(%r15)
> + larl %r4,function_trace_op
> + lg %r4,0(%r4)
> + lgr %r5, %r15
> + aghi %r5, 160
> + larl %r14,ftrace_trace_function
> + lg %r14,0(%r14)
> + basr %r14,%r14
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> + lg %r2,344(%r15)
> + lg %r3,448(%r15)
> +ENTRY(ftrace_regs_graph_caller)
> +# The bras instruction gets runtime patched to call prepare_ftrace_return.
> +# See ftrace_enable_ftrace_graph_caller. The patched instruction is:
> +# bras %r14,prepare_ftrace_return
> + bras %r14,0f
> +0: stg %r2,344(%r15)
> +#endif
> + lmg %r0,%r13,184(%r15)
> + lg %r14,176(%r15)
> + aghi %r15,336
> + br %r14
> +#endif

Some objections: this code assumes that sizeof(struct pt_regs) does not
change, which is not correct. So as soon as we touch struct pt_regs this
code would be broken. Also the order of the members within struct pt_regs
is not necessarily static (pt_regs is not ABI).

So using the supplied asm-offsets.c offsets within the pt_regs structure
would be the way to go.

In addition I don't like the code duplication. This is nearly an identical
copy of ftrace_caller, except that it (partially) creates a pt_regs structure
on the stack. I'd rather change the existing ftrace_caller code to do that
unconditionally.

However, what I _really_ do not like is the odd usage of r14 to create
a malformed psw member within the pt_regs structure, and thus omitting r14
from the gprs array.