Re: [PATCH 1/7] linux-user/nios2: Fix clone child return

[Richard Henderson]

On 3/25/22 05:49, Peter Maydell wrote:
> On Sun, 20 Mar 2022 at 16:12, Richard Henderson
> <richard.henderson@linaro.org> wrote:
> > The child side of clone needs to set the secondary
> > syscall return value, r7, to indicate syscall success.
> >
> > Advance the pc before do_syscall, so that the new thread
> > does not re-execute the clone syscall.
> >
> > Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> > ---
> >   linux-user/nios2/target_cpu.h | 1 +
> >   linux-user/nios2/cpu_loop.c   | 4 +---
> >   2 files changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/linux-user/nios2/target_cpu.h b/linux-user/nios2/target_cpu.h
> > index 2d2008f002..830b4c0741 100644
> > --- a/linux-user/nios2/target_cpu.h
> > +++ b/linux-user/nios2/target_cpu.h
> > @@ -27,6 +27,7 @@ static inline void cpu_clone_regs_child(CPUNios2State *env, 
> > target_ulong newsp,
> >           env->regs[R_SP] = newsp;
> >       }
> >       env->regs[R_RET0] = 0;
> > +    env->regs[7] = 0;
> >   }
> >
> >   static inline void cpu_clone_regs_parent(CPUNios2State *env, unsigned flags)
> > diff --git a/linux-user/nios2/cpu_loop.c b/linux-user/nios2/cpu_loop.c
> > index 1e93ef34e6..a3acaa92ca 100644
> > --- a/linux-user/nios2/cpu_loop.c
> > +++ b/linux-user/nios2/cpu_loop.c
> > @@ -42,8 +42,7 @@ void cpu_loop(CPUNios2State *env)
> >           case EXCP_TRAP:
> >               switch (env->error_code) {
> >               case 0:
> > -                qemu_log_mask(CPU_LOG_INT, "\nSyscall\n");
> > -
>
> Are you deliberately dropping this logging? If so, at least
> mention it in the commit message, but it doesn't really seem
> related to the rest of the patch...

It was intentional, but I meant to do it separately.

> > @@ -56,7 +55,6 @@ void cpu_loop(CPUNios2State *env)
> >                   env->regs[2] = abs(ret);
> >                   /* Return value is 0..4096 */
> >                   env->regs[7] = ret > 0xfffff000u;
> > -                env->regs[R_PC] += 4;
> >                   break;
>
> It feels a bit odd to be advancing the PC in the cpu-loop, because
> on the real hardware you get this for free because 'trap' sets
> ea to PC+4 and you just return to ea. But it works, I guess.

I thought of this in relation to the other trap patch set.  I think we should indeed raise 
the exception with the pc already advanced, as per hw.  This would avoid the need for 
nios2_cpu_do_interrupt to do it, except in the case of EXCP_IRQ.

But at present the translator is raising EXCP_TRAP with pc = trap insn.

> (The nios2 "use r2 and r7 for syscall return information" API
> seems like an unnecessary use of the architectural weirdness
> budget on their part, but whatever.)

Indeed.


r~