[dpdk-dev] [PATCH v5 4/5] lib/bpf: use wait event scheme for Rx/Tx iteration

[Ananyev, Konstantin]

> Hi Feifei,
> 
> > > Instead of polling for cbi->use to be updated, use wait event scheme.
> > >
> > > Furthermore, delete 'const' for 'bpf_eth_cbi_wait'. This is because of a
> > > compilation error:
> > > -----------------------------------------------------------------------
> > > ../lib/eal/include/rte_common.h:36:13: error: read-only variable ‘value’
> > > used as ‘asm’ output
> > >    36 | #define asm __asm__
> > >       |             ^~~~~~~
> > >
> > > ../lib/eal/arm/include/rte_pause_64.h:66:3: note: in expansion of macro ‘asm’
> > >    66 |   asm volatile("ldaxr %w[tmp], [%x[addr]]" \
> > >       |   ^~~
> > >
> > > ../lib/eal/arm/include/rte_pause_64.h:96:3: note: in expansion of macro
> > > ‘__LOAD_EXC_32’
> > >    96 |   __LOAD_EXC_32((src), dst, memorder)     \
> > >       |   ^~~~~~~~~~~~~
> > >
> > > ../lib/eal/arm/include/rte_pause_64.h:167:4: note: in expansion of macro
> > > ‘__LOAD_EXC’
> > >   167 |    __LOAD_EXC((addr), value, memorder, size) \
> > >       |    ^~~~~~~~~~
> > >
> > > ../lib/bpf/bpf_pkt.c:125:3: note: in expansion of macro ‘rte_wait_event’
> > >   125 |   rte_wait_event(&cbi->use, UINT32_MAX, ==, puse,
> > > -----------------------------------------------------------------------
> > >
> > > Signed-off-by: Feifei Wang <feifei.wang2 at arm.com>
> > > Reviewed-by: Ruifeng Wang <ruifeng.wang at arm.com>
> > > ---
> > >  lib/bpf/bpf_pkt.c | 11 ++++-------
> > >  1 file changed, 4 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/lib/bpf/bpf_pkt.c b/lib/bpf/bpf_pkt.c index
> > > 6e8248f0d6..213d44a75a 100644
> > > --- a/lib/bpf/bpf_pkt.c
> > > +++ b/lib/bpf/bpf_pkt.c
> > > @@ -111,9 +111,9 @@ bpf_eth_cbi_unuse(struct bpf_eth_cbi *cbi)
> > >   * Waits till datapath finished using given callback.
> > >   */
> > >  static void
> > > -bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi)
> > > +bpf_eth_cbi_wait(struct bpf_eth_cbi *cbi)
> >
> > Hi, Konstantin
> >
> > For this bpf patch, I delete 'const' through this is contrary to what we
> > discussed earlier. This is because if  we keep 'constant' here and use 'rte_wait_event'
> > new macro, compiler will report error. And earlier the arm version cannot be compiled
> > due to I forgot enable "wfe" config in the meson file, so this issue can not happen before.
> 
> 
> Honestly, I don't understand why we have to remove perfectly valid 'const' qualifier here.
> If this macro can't be used with pointers to const (still don't understand why),
> then let's just not use this macro here.
> Strictly speaking I don't see much benefit here from it.
> 
> >
> > >  {
> > > -	uint32_t nuse, puse;
> > > +	uint32_t puse;
> > >
> > >  	/* make sure all previous loads and stores are completed */
> > >  	rte_smp_mb();
> > > @@ -122,11 +122,8 @@ bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi)
> > >
> > >  	/* in use, busy wait till current RX/TX iteration is finished */
> > >  	if ((puse & BPF_ETH_CBI_INUSE) != 0) {
> > > -		do {
> > > -			rte_pause();
> > > -			rte_compiler_barrier();
> > > -			nuse = cbi->use;
> > > -		} while (nuse == puse);
> > > +		rte_wait_event(&cbi->use, UINT32_MAX, ==, puse,
> > > +				__ATOMIC_RELAXED);

After another thought, if we do type conversion at macro invocation time:

bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi)
{
  ...
  rte_wait_event((uint32_t *)&cbi->use, UINT32_MAX, ==, puse, __ATOMIC_RELAXED);

would that help?


> > >  	}
> > >  }
> > >
> > > --
> > > 2.25.1