Re: [Qemu-devel] [RFC 19/38] tcg: add tcg_gen_smp_rmb()

[Emilio G. Cota]

On Thu, Sep 10, 2015 at 17:01:14 +0100, Alex Bennée wrote:
> 
> Emilio G. Cota <address@hidden> writes:
> 
> > Signed-off-by: Emilio G. Cota <address@hidden>
> > ---
> >  tcg/tcg-op.h | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/tcg/tcg-op.h b/tcg/tcg-op.h
> > index 52482c0..3ec9f13 100644
> > --- a/tcg/tcg-op.h
> > +++ b/tcg/tcg-op.h
> > @@ -716,6 +716,16 @@ static inline void tcg_gen_fence_full(void)
> >      tcg_gen_op0(&tcg_ctx, INDEX_op_fence_full);
> >  }
> >  
> > +#if defined(__i386__) || defined(__x86_64__) || defined(__s390x__)
> > +static inline void tcg_gen_smp_rmb(void)
> > +{ }
> > +#else
> > +static inline void tcg_gen_smp_rmb(void)
> > +{
> > +    tcg_gen_fence_load();
> > +}
> > +#endif
> 
> This seems a little pointless wrapping up tcg_gen_fence_load. Could the
> magic dealing with the backend not be done with something like
> TCG_TARGET_HAS_fence_load. On the x86/x86_64 backends this could then
> NOP away.

This patch made sense at the time as a companion to this other patch:

cpu: add barriers around cpu->tcg_exit_req
(snip)
+++ b/include/exec/gen-icount.h                                                 
                                                                                
                                                                                
                                            
@@ -16,6 +16,7 @@ static inline void gen_tb_start(TranslationBlock *tb)         
                                                                                
                                                                                
                                            
                                                                                
                                                                                
                                                                                
                                            
     exitreq_label = gen_new_label();                                           
                                                                                
                                                                                
                                            
     flag = tcg_temp_new_i32();                                                 
                                                                                
                                                                                
                                            
+    tcg_gen_smp_rmb();                                                         
                                                                                
                                                                                
                                            
     tcg_gen_ld_i32(flag, cpu_env,                                              
                                                                                
                                                                                
                                            
                    offsetof(CPUState, tcg_exit_req) - ENV_OFFSET);             
                                                                                
                                                                                
                                            
     tcg_gen_brcondi_i32(TCG_COND_NE, flag, 0, exitreq_label);

Paolo had the better idea of calling smp_rmb() once we've
exited TCG, which renders tcg_gen_smp_rmb() unnecessary.

                Emilio