Re: [PATCH v2 1/2] tracing: fprobe: optimization for entry only case

[Google]

Hi Menglong,

I remember why I haven't implement this.

On Fri, 10 Oct 2025 11:38:46 +0800
Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:

> +
> +static struct ftrace_ops fprobe_ftrace_ops = {
> +	.func	= fprobe_ftrace_entry,
> +	.flags	= FTRACE_OPS_FL_SAVE_REGS,

Actually, this flag is the problem. This can fail fprobe on architecture
which does not support CONFIG_DYNAMIC_FTRACE_WITH_REGS (e.g. arm64, riscv)

 * SAVE_REGS - The ftrace_ops wants regs saved at each function called
 *            and passed to the callback. If this flag is set, but the
 *            architecture does not support passing regs
 *            (CONFIG_DYNAMIC_FTRACE_WITH_REGS is not defined), then the
 *            ftrace_ops will fail to register, unless the next flag
 *            is set.

fgraph has a special entry code for saving ftrace_regs.
So at least we need to fail back to fgraph if arch does not
support CONFIG_DYNAMIC_FTRACE_WITH_REGS.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>