Re: [V3] net/qede: reduce the optimization level for gcc > 11

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 10 Sep 2025 08:49:38 +0200
Thierry Herbelot <thierry.herbelot@6wind.com> wrote:

> Hello Stephen,
> 
> On 9/9/25 18:17, Stephen Hemminger wrote:
> > On Tue,  9 Sep 2025 07:40:23 +0200
> > Thierry Herbelot <thierry.herbelot@6wind.com> wrote:
> >   
> >> The qede PMD stopped working under Ubuntu-24.04 (using gcc-13) when
> >> compiled with -O3 (default level for all DPDK code). A bug is opened
> >> for this issue (see Link). The same issue is also seen with gcc-12
> >> and gcc-14. The issue is not seen with clang-20.
> >>
> >> A first workaround is to just disable all optimizations (-O0), which
> >> restores packet Rx with Qlogic NICs. However, the performance impact
> >> is not acceptable (around 50% drop).
> >>
> >> A better compromise is to use -O1 for the qede PMD:
> >> - there is some perf impact,
> >> - but the PMD is working as expected (packets are correctly received).
> >>
> >> When compiling with both -O2 and -O3 and gcc > 11, there is no packet
> >> reception when using the qede PMD. The root cause could be missing
> >> 'volatile' keywords or missing memory barriers in the qede PMD code.
> >>
> >> Link: https://bugs.dpdk.org/show_bug.cgi?id=1379
> >> Signed-off-by: Thierry Herbelot <thierry.herbelot@6wind.com>  
> > 
> > Which rx_burst gets used on that hardware?
> > 
> > Is there any indication of errors (like rx_mbuf_alloc_failed) in the stats.
> > 
> > The driver does not appear to have any write barrier after updating the consumed packets.
> > 
> > Would this help?
> > diff --git a/drivers/net/qede/qede_rxtx.c b/drivers/net/qede/qede_rxtx.c
> > index 25e28fd9f6..1b5109d966 100644
> > --- a/drivers/net/qede/qede_rxtx.c
> > +++ b/drivers/net/qede/qede_rxtx.c
> > @@ -1295,6 +1295,8 @@ static inline void qede_rx_bd_ring_consume(struct qede_rx_queue *rxq)
> >   {
> >          ecore_chain_consume(&rxq->rx_bd_ring);
> >          rxq->sw_rx_cons++;
> > +
> > +       rte_wmb();
> >   }
> >     
> I just checked with this patch, but the error is still present:
> 
> testpmd> show fwd stats all  
> 
>    ---------------------- Forward statistics for port 0 
> ----------------------
>    RX-packets: 0              RX-dropped: 10            RX-total: 10
>    RX-error: 0
>    RX-nombufs: 10
>    TX-packets: 0              TX-dropped: 0             TX-total: 0
>   
> ----------------------------------------------------------------------------
> 
> The error chain is:
> 
> RX-nombufs
>   => (testpmd) stats.rx_nombuf
>    => (qede) p_stats->common.no_buff_discards
>     => (qede/base) HILO_64_REGPAIR(mstats.no_buff_discard)  
>           (internal to the NIC)
> 

Thanks, lets root cause this (rather than paper it over) because
other drivers may have same issue. Since no mbufs are present
looks like rx discriptor never made it to NIC