Re: [patches] [PATCH v2 2/6] riscv/ftrace: Add dynamic function tracer support

[Alan Kao <nonerkao@gmail.com>]

On Sun, Jan 14, 2018 at 11:24:53PM -0800, Stefan O'Rear wrote:
> On Sun, Jan 14, 2018 at 10:47 PM, Alan Kao <nonerkao@gmail.com> wrote:
> > +       /*
> > +        * For the dynamic ftrace to work, here we should reserve at least
> > +        * 8 bytes for a functional auipc-jalr pair. Pseudo inst nop may be
> > +        * interpreted as different length in different models, so we manually
> > +        * *expand* two 4-byte nops here.
> > +        *
> > +        * Calling ftrace_update_ftrace_func would overwrite the nops below.
> > +        * Check ftrace_modify_all_code for details.
> > +        */
> > +       addi    x0, x0, 0
> > +       addi    x0, x0, 0
> 
> This relies on behavior of the assembler which is undocumented and, if
> my reading of the specification is correct, a bug.
> 
> The documented way to assemble an sequence of 2 4-byte NOPs regardless
> of subtarget is as follows:
> 
> .option push
> .option norvc
>     nop
>     nop
> .option pop
> 
> I have filed https://github.com/riscv/riscv-binutils-gdb/issues/135 to
> get clarity on the assembler behavior; the explicit approach may be
> preferable even if the assembler behavior turns out to be correct.
> 
> -s
> 

Thanks for pointing this out.

After checking the way other architectures have done this, I think we can 
just put a 

	call ftrace_stub

here.  Currently we don't support linker relaxing with ftrace, so this
macro will be expand to 8-byte inst-pair.

Alan

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