Re: [PATCH v4 22/24] target/ppc: fix timebase register reset state

[Nicholas Piggin]

On Tue Mar 12, 2024 at 11:24 PM AEST, Alex Bennée wrote:
> Nicholas Piggin <npiggin@gmail.com> writes:
>
> > (H)DEC and PURR get reset before icount does, which causes them to be
> > skewed and not match the init state. This can cause replay to not
> > match the recorded trace exactly. For DEC and HDEC this is usually not
> > noticable since they tend to get programmed before affecting the
> > target machine. PURR has been observed to cause replay bugs when
> > running Linux.
> >
> > Fix this by resetting using a time of 0.
> >
> > Cc: qemu-ppc@nongnu.org
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > ---
> >  hw/ppc/ppc.c         | 11 ++++++++---
> >  target/ppc/machine.c |  4 ++++
> >  2 files changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c
> > index fadb8f5239..f22321779e 100644
> > --- a/hw/ppc/ppc.c
> > +++ b/hw/ppc/ppc.c
> > @@ -1112,16 +1112,21 @@ void cpu_ppc_tb_reset(CPUPPCState *env)
> >          timer_del(tb_env->hdecr_timer);
> >          ppc_set_irq(cpu, PPC_INTERRUPT_HDECR, 0);
> >          tb_env->hdecr_next = 0;
> > +        _cpu_ppc_store_hdecr(cpu, 0, 0, 0, 64);
> >      }
> >  
> >      /*
> >       * There is a bug in Linux 2.4 kernels:
> >       * if a decrementer exception is pending when it enables msr_ee at 
> > startup,
> >       * it's not ready to handle it...
> > +     *
> > +     * On machine reset, in this is called before icount is reset, so
> > +     * for icount-mode, setting TB registers using now=qemu_clock_get_ns
> > +     * results in them being skewed when icount does get reset. Use an
> > +     * explicit 0 to get a consistent reset state.
> >       */
> > -    cpu_ppc_store_decr(env, -1);
> > -    cpu_ppc_store_hdecr(env, -1);
> > -    cpu_ppc_store_purr(env, 0x0000000000000000ULL);
> > +    _cpu_ppc_store_decr(cpu, 0, 0, -1, 64);
> > +    _cpu_ppc_store_purr(env, 0, 0);
> >  }
> >  
> >  void cpu_ppc_tb_free(CPUPPCState *env)
> > diff --git a/target/ppc/machine.c b/target/ppc/machine.c
> > index 203fe28e01..4c4294eafe 100644
> > --- a/target/ppc/machine.c
> > +++ b/target/ppc/machine.c
> > @@ -215,6 +215,8 @@ static int cpu_pre_save(void *opaque)
> >           * it here.
> >           */
> >          env->spr[SPR_DECR] = cpu_ppc_load_decr(env);
> > +        printf("cpu_ppc_pre_save  TB:0x%016lx\n", cpu_ppc_load_tbl(env));
> > +        printf("cpu_ppc_pre_save DEC:0x%016lx\n",
> >  cpu_ppc_load_decr(env));
>
> I think this is left over debug, which btw needs proper types:

Yes you're right, sorry that was intended to be removed.

Thanks,
Nick