RE: [RFC 2/4] gro: remove use of VLAs

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> >> On 5/23/2024 5:26 PM, Konstantin Ananyev wrote:
> >>> From: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> >>>
> >>> ../lib/gro/rte_gro.c:182:34: warning: variable length array used [-Wvla]
> >>> ../lib/gro/rte_gro.c:363:34: warning: variable length array used [-Wvla]
> >>>
> >>> In both cases the pattern is the same: we use unprocess_pkts[nb_pkts] to
> >>> collect un-used by GRO packets, and then copy them to the start of
> >>> input/output pkts[] array.
> >>> In both cases, we can safely copy pkts[i] into already
> >>> processed entry at the same array, i.e. into pkts[unprocess_num].
> >>> Such change eliminates need of temporary VLA: unprocess_pkts[nb_pkts].
> >>>
> >>> Signed-off-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> >>> ---
> >>>  lib/gro/rte_gro.c | 40 ++++++++++++++--------------------------
> >>>  1 file changed, 14 insertions(+), 26 deletions(-)
> >>>
> >>> diff --git a/lib/gro/rte_gro.c b/lib/gro/rte_gro.c
> >>> index db86117609..6d5aadf32a 100644
> >>> --- a/lib/gro/rte_gro.c
> >>> +++ b/lib/gro/rte_gro.c
> >>> @@ -179,7 +179,6 @@ rte_gro_reassemble_burst(struct rte_mbuf **pkts,
> >>>  	struct gro_vxlan_udp4_item vxlan_udp_items[RTE_GRO_MAX_BURST_ITEM_NUM]
> >>>  			= {{{0}} };
> >>>
> >>> -	struct rte_mbuf *unprocess_pkts[nb_pkts];
> >>>  	uint32_t item_num;
> >>>  	int32_t ret;
> >>>  	uint16_t i, unprocess_num = 0, nb_after_gro = nb_pkts;
> >>> @@ -275,7 +274,7 @@ rte_gro_reassemble_burst(struct rte_mbuf **pkts,
> >>>  				/* Merge successfully */
> >>>  				nb_after_gro--;
> >>>  			else if (ret < 0)
> >>> -				unprocess_pkts[unprocess_num++] = pkts[i];
> >>> +				pkts[unprocess_num++] = pkts[i];
> >>>  		} else if (IS_IPV4_VXLAN_UDP4_PKT(pkts[i]->packet_type) &&
> >>>  				do_vxlan_udp_gro) {
> >>>  			ret = gro_vxlan_udp4_reassemble(pkts[i],
> >>> @@ -284,7 +283,7 @@ rte_gro_reassemble_burst(struct rte_mbuf **pkts,
> >>>  				/* Merge successfully */
> >>>  				nb_after_gro--;
> >>>  			else if (ret < 0)
> >>> -				unprocess_pkts[unprocess_num++] = pkts[i];
> >>> +				pkts[unprocess_num++] = pkts[i];
> >>>  		} else if (IS_IPV4_TCP_PKT(pkts[i]->packet_type) &&
> >>>  				do_tcp4_gro) {
> >>>  			ret = gro_tcp4_reassemble(pkts[i], &tcp_tbl, 0);
> >>> @@ -292,7 +291,7 @@ rte_gro_reassemble_burst(struct rte_mbuf **pkts,
> >>>  				/* merge successfully */
> >>>  				nb_after_gro--;
> >>>  			else if (ret < 0)
> >>> -				unprocess_pkts[unprocess_num++] = pkts[i];
> >>> +				pkts[unprocess_num++] = pkts[i];
> >>>  		} else if (IS_IPV4_UDP_PKT(pkts[i]->packet_type) &&
> >>>  				do_udp4_gro) {
> >>>  			ret = gro_udp4_reassemble(pkts[i], &udp_tbl, 0);
> >>> @@ -300,7 +299,7 @@ rte_gro_reassemble_burst(struct rte_mbuf **pkts,
> >>>  				/* merge successfully */
> >>>  				nb_after_gro--;
> >>>  			else if (ret < 0)
> >>> -				unprocess_pkts[unprocess_num++] = pkts[i];
> >>> +				pkts[unprocess_num++] = pkts[i];
> >>>  		} else if (IS_IPV6_TCP_PKT(pkts[i]->packet_type) &&
> >>>  				do_tcp6_gro) {
> >>>  			ret = gro_tcp6_reassemble(pkts[i], &tcp6_tbl, 0);
> >>> @@ -308,21 +307,15 @@ rte_gro_reassemble_burst(struct rte_mbuf **pkts,
> >>>  				/* merge successfully */
> >>>  				nb_after_gro--;
> >>>  			else if (ret < 0)
> >>> -				unprocess_pkts[unprocess_num++] = pkts[i];
> >>> +				pkts[unprocess_num++] = pkts[i];
> >>>  		} else
> >>> -			unprocess_pkts[unprocess_num++] = pkts[i];
> >>> +			pkts[unprocess_num++] = pkts[i];
> >>>  	}
> >>>
> >>>  	if ((nb_after_gro < nb_pkts)
> >>>  		 || (unprocess_num < nb_pkts)) {
> >>> -		i = 0;
> >>> -		/* Copy unprocessed packets */
> >>> -		if (unprocess_num > 0) {
> >>> -			memcpy(&pkts[i], unprocess_pkts,
> >>> -					sizeof(struct rte_mbuf *) *
> >>> -					unprocess_num);
> >>> -			i = unprocess_num;
> >>> -		}
> >>> +
> >>> +		i = unprocess_num;
> >>>
> >>>  		/* Flush all packets from the tables */
> >>>  		if (do_vxlan_tcp_gro) {
> >>>
> >>
> >> ack to re-use 'pkts[]' buffer for unprocessed packets, that should work.
> >>
> >> But as a more general GRO question, above 'rte_gro_reassemble_burst()'
> >> functions seems returns 'nb_after_gro' and as far as I can see that
> >> amount of mbufs sits in the 'pkts[]'.
> >> When packets flushed from tables, flushed packets are replaced to
> >> 'pkts[]' but still 'nb_after_gro' returned, there is no way for
> >> application to know that more than 'nb_after_gro' mbufs available in the
> >> 'pkts[]'. Shouldn't return value increased per flushed packet?
> >>
> >> Ahh, I can see it was the case before, but it is updated (perhaps
> >> broken) in commit:
> >> 74080d7dcf31 ("gro: support IPv6 for TCP")
> >
> > Actually my first thought was - we should return 'I' here.
> > but then looking at the code more carefully, I realized that it is correct:
> > nb_after_gro - would contain valid number of packets
> > (at least I wasn't able to find a case when it wouldn't).
> > Though yeh, it wasn't very obvious for me at first place, so might be
> > extra comment wouldn't hurt here.
> >
> 
> In first half of the function, 'nb_after_gro' is number of packets not
> assembled and decided to pass back to user via 'pkts' buffer.
> 
> In second half, timed out packets are decided to turn back to user
> (flushed), as they are not reassembled, and these packets are added to
> 'pkts' array for user, but 'nb_after_gro' not increased. So how user can
> know about it?
> 
> Basically, I think we should return 'i', what am I missing, can you
> please detail?

Actually, as I understand the logic is different from what you described above.
At the start   nb_after_gro equals to total number of input packets:
 nb_after_gro = nb_pkts;
Then later, for each packet that was merged with some other packet it decrements:
ret = gro_..._reassemble(pkts[i], ...);
if (ret > 0)
      /* Merge successfully */
     nb_after_gro--;

So at the end nb_after_gro contains number of input packets minus number
of packets that were merged.
Which, as I undersrand should be equal to 'I' value.
So, no change here is necessary, I think.
Except probably some extra comment to avoid confusion.