Re: ftrace for MIPS

[Sergei Shtylyov <sshtylyov@xxxxxxxxxxxxx>]

Hello.

Steven Rostedt wrote:

> > Need to check which registers is missing when saving/restoring for
> > _mcount:

> > NESTED(ftrace_graph_caller, PT_SIZE, ra) 
> >        MCOUNT_SAVE_REGS
> >        PTR_S   v0, PT_R2(sp)
> >
> >        MCOUNT_SET_ARGS
> >        jal     prepare_ftrace_return
> >        nop
> >
> >        /* overwrite the parent as &return_to_handler: v0 -> $1(at) */
> >        move    $1,     v0  

> I'm confused here? I'm not exactly sure what the above is doing. Is $1 a
> register (AT)?

   Yes.

> And how is this register used before calling mcount?

> >        PTR_L   v0, PT_R2(sp)
> >        MCOUNT_RESTORE_REGS
> >        RETURN_BACK
> >        END(ftrace_graph_caller)

> >        .align  2
> >        .globl  return_to_handler
> > return_to_handler:
> >        PTR_SUBU        sp, PT_SIZE
> >        PTR_S   v0, PT_R2(sp)

> BTW, is v0 the only return register? I know x86 can return two different
> registers depending on what it returns. What happens if a function
> returns a 64 bit value on a 32bit box? Does it use two registers for
> that?

   Yes, there's also v1 register.

WBR, Sergei