Re: [dpdk-dev] [PATCH v3 2/2] net/i40e: replace SMP barrier with thread fence

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> > > >
> > > > Simply replace the SMP barrier with atomic thread fence for i40e
> > > > hw ring
> > sacn,
> > > > if there is no synchronization point.
> > > >
> > > > Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> > > > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > > ---
> > > >  drivers/net/i40e/i40e_rxtx.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/net/i40e/i40e_rxtx.c
> > > > b/drivers/net/i40e/i40e_rxtx.c index 9aaabfd92..86e2f083e 100644
> > > > --- a/drivers/net/i40e/i40e_rxtx.c
> > > > +++ b/drivers/net/i40e/i40e_rxtx.c
> > > > @@ -482,7 +482,8 @@ i40e_rx_scan_hw_ring(struct i40e_rx_queue
> > > > *rxq)
> > > >
> > I40E_RXD_QW1_STATUS_SHIFT;
> > > >               }
> > > >
> > > > -             rte_smp_rmb();
> > > > +             /* This barrier is to order loads of different words
> > > > + in the
> > descriptor */
> > > > +             rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
> > >
> > > Now for x86, you actually replace a compiler barrier with a memory
> > > fence,
> > this may have potential performance impact which need additional
> > resource to investigate
> >
> > No memory fence instruction is generated for __ATOMIC_ACQUIRE on x86
> > for any version of gcc or clang that I've tried, based on experiments
> > here:
> >
> >     https://godbolt.org/z/Yxr1vGhKP
> 
> Nice tool!
> I try to write some dummy code combined with or without
> __atomic_thread_fence(__ATOMIC_ACQUIRE)
> but I didn't see any difference of the generated assembly code, does that means
> __atomic_thread_fence(__ATOMIC_ACQUIRE) just does nothing on x86?
Yes, it should not have any barriers generated for x86. At the same time it also acts as a compiler barrier.