Re: [PATCH v3 01/33] target/ppc: Fix gen_sc to use correct nip

["Nicholas Piggin" <npiggin@gmail.com>]

On Thu May 9, 2024 at 1:17 AM AEST, BALATON Zoltan wrote:
> On Wed, 8 May 2024, Nicholas Piggin wrote:
> > On Wed May 8, 2024 at 10:14 AM AEST, BALATON Zoltan wrote:
> >> Most exceptions are raised with nip pointing to the faulting
> >> instruction but the sc instruction generating a syscall exception
> >> leaves nip pointing to next instruction. Fix gen_sc to not use
> >> gen_exception_err() which sets nip back but correctly set nip to
> >> pc_next so we don't have to patch this in the exception handlers.
> >>
> >> Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
> >> ---
> >>  target/ppc/excp_helper.c | 43 ++--------------------------------------
> >>  target/ppc/translate.c   | 15 ++++++--------
> >>  2 files changed, 8 insertions(+), 50 deletions(-)
> >>
> >> diff --git a/target/ppc/excp_helper.c b/target/ppc/excp_helper.c
> >> index 0712098cf7..92fe535815 100644
> >> --- a/target/ppc/excp_helper.c
> >> +++ b/target/ppc/excp_helper.c
> >> @@ -116,7 +116,7 @@ static void dump_syscall(CPUPPCState *env)
> >>                    ppc_dump_gpr(env, 0), ppc_dump_gpr(env, 3),
> >>                    ppc_dump_gpr(env, 4), ppc_dump_gpr(env, 5),
> >>                    ppc_dump_gpr(env, 6), ppc_dump_gpr(env, 7),
> >> -                  ppc_dump_gpr(env, 8), env->nip);
> >> +                  ppc_dump_gpr(env, 8), env->nip - 4);
> >>  }
> >>
> >>  static void dump_hcall(CPUPPCState *env)
> >> @@ -131,7 +131,7 @@ static void dump_hcall(CPUPPCState *env)
> >>                    ppc_dump_gpr(env, 7), ppc_dump_gpr(env, 8),
> >>                    ppc_dump_gpr(env, 9), ppc_dump_gpr(env, 10),
> >>                    ppc_dump_gpr(env, 11), ppc_dump_gpr(env, 12),
> >> -                  env->nip);
> >> +                  env->nip - 4);
> >>  }
> >>
> >>  #ifdef CONFIG_TCG
> >> @@ -516,12 +516,6 @@ static void powerpc_excp_40x(PowerPCCPU *cpu, int excp)
> >>          break;
> >>      case POWERPC_EXCP_SYSCALL:   /* System call exception                    */
> >>          dump_syscall(env);
> >> -
> >> -        /*
> >> -         * We need to correct the NIP which in this case is supposed
> >> -         * to point to the next instruction
> >> -         */
> >> -        env->nip += 4;
> >>          break;
> >>      case POWERPC_EXCP_FIT:       /* Fixed-interval timer interrupt           */
> >>          trace_ppc_excp_print("FIT");
> >> @@ -632,12 +626,6 @@ static void powerpc_excp_6xx(PowerPCCPU *cpu, int excp)
> >>          break;
> >>      case POWERPC_EXCP_SYSCALL:   /* System call exception                    */
> >>          dump_syscall(env);
> >> -
> >> -        /*
> >> -         * We need to correct the NIP which in this case is supposed
> >> -         * to point to the next instruction
> >> -         */
> >> -        env->nip += 4;
> >>          break;
> >>      case POWERPC_EXCP_FPU:       /* Floating-point unavailable exception     */
> >>      case POWERPC_EXCP_DECR:      /* Decrementer exception                    */
> >> @@ -780,13 +768,6 @@ static void powerpc_excp_7xx(PowerPCCPU *cpu, int excp)
> >>          } else {
> >>              dump_syscall(env);
> >>          }
> >> -
> >> -        /*
> >> -         * We need to correct the NIP which in this case is supposed
> >> -         * to point to the next instruction
> >> -         */
> >> -        env->nip += 4;
> >> -
> >>          /*
> >>           * The Virtual Open Firmware (VOF) relies on the 'sc 1'
> >>           * instruction to communicate with QEMU. The pegasos2 machine
> >> @@ -932,13 +913,6 @@ static void powerpc_excp_74xx(PowerPCCPU *cpu, int excp)
> >>          } else {
> >>              dump_syscall(env);
> >>          }
> >> -
> >> -        /*
> >> -         * We need to correct the NIP which in this case is supposed
> >> -         * to point to the next instruction
> >> -         */
> >> -        env->nip += 4;
> >> -
> >>          /*
> >>           * The Virtual Open Firmware (VOF) relies on the 'sc 1'
> >>           * instruction to communicate with QEMU. The pegasos2 machine
> >> @@ -1098,12 +1072,6 @@ static void powerpc_excp_booke(PowerPCCPU *cpu, int excp)
> >>          break;
> >>      case POWERPC_EXCP_SYSCALL:   /* System call exception                    */
> >>          dump_syscall(env);
> >> -
> >> -        /*
> >> -         * We need to correct the NIP which in this case is supposed
> >> -         * to point to the next instruction
> >> -         */
> >> -        env->nip += 4;
> >>          break;
> >>      case POWERPC_EXCP_FPU:       /* Floating-point unavailable exception     */
> >>      case POWERPC_EXCP_APU:       /* Auxiliary processor unavailable          */
> >> @@ -1428,13 +1396,6 @@ static void powerpc_excp_books(PowerPCCPU *cpu, int excp)
> >>          } else {
> >>              dump_syscall(env);
> >>          }
> >> -
> >> -        /*
> >> -         * We need to correct the NIP which in this case is supposed
> >> -         * to point to the next instruction
> >> -         */
> >> -        env->nip += 4;
> >> -
> >>          /* "PAPR mode" built-in hypercall emulation */
> >>          if (lev == 1 && books_vhyp_handles_hcall(cpu)) {
> >>              PPCVirtualHypervisorClass *vhc =
> >> diff --git a/target/ppc/translate.c b/target/ppc/translate.c
> >> index 93ffec787c..e112c44a02 100644
> >> --- a/target/ppc/translate.c
> >> +++ b/target/ppc/translate.c
> >> @@ -4472,22 +4472,19 @@ static void gen_hrfid(DisasContext *ctx)
> >>  #endif
> >>
> >>  /* sc */
> >> -#if defined(CONFIG_USER_ONLY)
> >> -#define POWERPC_SYSCALL POWERPC_EXCP_SYSCALL_USER
> >> -#else
> >> -#define POWERPC_SYSCALL POWERPC_EXCP_SYSCALL
> >> -#endif
> >>  static void gen_sc(DisasContext *ctx)
> >>  {
> >> -    uint32_t lev;
> >> -
> >>      /*
> >>       * LEV is a 7-bit field, but the top 6 bits are treated as a reserved
> >>       * field (i.e., ignored). ISA v3.1 changes that to 5 bits, but that is
> >>       * for Ultravisor which TCG does not support, so just ignore the top 6.
> >>       */
> >> -    lev = (ctx->opcode >> 5) & 0x1;
> >> -    gen_exception_err(ctx, POWERPC_SYSCALL, lev);
> >> +    uint32_t lev = (ctx->opcode >> 5) & 0x1;
> >> +#ifdef CONFIG_USER_ONLY
> >> +    gen_exception_err(ctx, POWERPC_EXCP_SYSCALL_USER, lev);
> >> +#else
> >> +    gen_exception_err_nip(ctx, POWERPC_EXCP_SYSCALL, lev, ctx->base.pc_next);
> >> +#endif
> >
> > I think this is the nail in the coffin for this one. Let's shelve it.
>
> I really would like to get rid of all the +4s and long comments in 
> excp_helper.c though so I won't let this go until we find a solution. I've 

All the += 4 are just because the handlers are repeated so many times.
And with this you have to add the - 4 for other cases.

And you removed the comment but you didn't add one here. Comments aren't
really a problem.

> now found that linux-user/ppc/cpu_loop.c handles this case and that also 
> has a +4 that I've missed before so with that removed this should work. 
> I'll try again.

The invariant that nip always refers to the instruction that caused the
exception for synchronous exceptions is a good one, and the patch has
caused a bunch of problems, so I don't want to merge it.

Thanks,
Nick