Re: [dpdk-dev] [PATCH] app/testpmd: fix random number of Tx segments

["Li, Xiaoyun" <xiaoyun.li@intel.com>]

> -----Original Message-----
> From: Zhang, AlvinX <alvinx.zhang@intel.com>
> Sent: Monday, September 6, 2021 18:04
> To: Li, Xiaoyun <xiaoyun.li@intel.com>; Ananyev, Konstantin
> <konstantin.ananyev@intel.com>
> Cc: dev@dpdk.org; stable@dpdk.org
> Subject: RE: [PATCH] app/testpmd: fix random number of Tx segments
> 
> > -----Original Message-----
> > From: Li, Xiaoyun <xiaoyun.li@intel.com>
> > Sent: Monday, September 6, 2021 4:59 PM
> > To: Zhang, AlvinX <alvinx.zhang@intel.com>; Ananyev, Konstantin
> > <konstantin.ananyev@intel.com>
> > Cc: dev@dpdk.org; stable@dpdk.org
> > Subject: RE: [PATCH] app/testpmd: fix random number of Tx segments
> >
> > Hi
> >
> > > -----Original Message-----
> > > From: Zhang, AlvinX <alvinx.zhang@intel.com>
> > > Sent: Thursday, September 2, 2021 16:20
> > > To: Li, Xiaoyun <xiaoyun.li@intel.com>; Ananyev, Konstantin
> > > <konstantin.ananyev@intel.com>
> > > Cc: dev@dpdk.org; Zhang, AlvinX <alvinx.zhang@intel.com>;
> > > stable@dpdk.org
> > > Subject: [PATCH] app/testpmd: fix random number of Tx segments
> > >
> > > When random number of segments in Tx packets is enabled, the total
> > > data space length of all segments must be greater or equal than the
> > > size of an Eth/IP/UDP/timestamp packet, that's total 14 + 20 + 8 +
> > > 16 bytes. Otherwise the Tx engine may cause the application to crash.
> > >
> > > Bugzilla ID: 797
> > > Fixes: 79bec05b32b7 ("app/testpmd: add ability to split outgoing
> > > packets")
> > > Cc: stable@dpdk.org
> > >
> > > Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
> > > ---
> > >  app/test-pmd/config.c  | 16 +++++++++++-----
> > > app/test-pmd/testpmd.c
> > > |  5
> > > +++++  app/test-pmd/testpmd.h |  5 +++++  app/test-pmd/txonly.c  |
> > > +++++ 7
> > > +++++ +++++--
> > >  4 files changed, 26 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/app/test-pmd/config.c b/app/test-pmd/config.c index
> > > 31d8ba1..5105b3b 100644
> > > --- a/app/test-pmd/config.c
> > > +++ b/app/test-pmd/config.c
> > > @@ -3837,10 +3837,11 @@ struct igb_ring_desc_16_bytes {
> > >  	 * Check that each segment length is greater or equal than
> > >  	 * the mbuf data size.
> > >  	 * Check also that the total packet length is greater or equal than the
> > > -	 * size of an empty UDP/IP packet (sizeof(struct rte_ether_hdr) +
> > > -	 * 20 + 8).
> > > +	 * size of an Eth/IP/UDP + timestamp packet
> > > +	 * (sizeof(struct rte_ether_hdr) + 20 + 8 + 16).
> >
> > I don't really agree on this. Most of the time, txonly generate
> > packets with Eth/IP/UDP. It's not fair to limit the hdr length to include
> timestamp in all cases.
> > And to be honest, I don't see why you need to add
> > "tx_pkt_nb_min_segs". It's only used in txonly when
> > "TX_PKT_SPLIT_RND". So this issue is because when "TX_PKT_SPLIT_RND", the
> random nb_segs is not enough for the hdr.
> >
> > But if you read txonly carefully, if "TX_PKT_SPLIT_RND", the first
> > segment length should be equal or greater than 42 (14+20+8). Because
> > when "TX_PKT_SPLIT_RND", update_pkt_header() should be called. And
> > that function doesn't deal with header in multi-segments.
> > I think there's bug here.
> >
> > So I think you should only add a check in pkt_burst_prepare() in txonly().
> > 	if (unlikely(tx_pkt_split == TX_PKT_SPLIT_RND) || txonly_multi_flow)
> > +                           if (tx_pkt_seg_lengths[0] < 42) {
> > +			err_log;
> > +			return false;
> > +		}
> > 		update_pkt_header(pkt, pkt_len);
> 
> Yes, I didn't notice the updating for the UDP header, but the bug first occurs in
> this function copy_buf_to_pkt(&pkt_udp_hdr, sizeof(pkt_udp_hdr), pkt,
> 			sizeof(struct rte_ether_hdr) +
> 			sizeof(struct rte_ipv4_hdr));
> not in update_pkt_header.
> 
> Here we expecting users should set minimum 42 byte for first segment seems ok,
> But I think we putting the check in configuring the data space length of first
> segment is more graceful.

No. You didn't get my point. It's not graceful at all.
The segment fault will only happen when "TX_PKT_SPLIT_RND". Because the hdr may take 2 segs while random nb_segs is only 1.
But if it's not "TX_PKT_SPLIT_RND", the current set_tx_pkt_segments() already make sure pkt_len is enough for 42.

So the only case you need to deal with is "TX_PKT_SPLIT_RND". And since "TX_PKT_SPLIT_RND" actually needs the first segment to be enough to contain 42 bytes.
And in cmd_set_txpkts_parsed, you may not get the configuration to know if it will be configured as "TX_PKT_SPLIT_RND". That's why you should check before update_pkt_header().

In this way, when it's NOT "TX_PKT_SPLIT_RND", it will maintain the old behavior that hdrs may cross several segs which makes more sense.

> 
> >
> > As for timestamp, maybe refer to "pkt_copy_split" in csumonly.c is
> > better? Copy the extra to the last segment if it's not enough. Not
> > sure how to deal with this issue better.
> >
> > >  	 */
> > >  	tx_pkt_len = 0;
> > > +	tx_pkt_nb_min_segs = 0;
> > >  	for (i = 0; i < nb_segs; i++) {
> > >  		if (seg_lengths[i] > mbuf_data_size[0]) {
> > >  			fprintf(stderr,
> > > @@ -3849,11 +3850,16 @@ struct igb_ring_desc_16_bytes {
> > >  			return;
> > >  		}
> > >  		tx_pkt_len = (uint16_t)(tx_pkt_len + seg_lengths[i]);
> > > +
> > > +		if (!tx_pkt_nb_min_segs &&
> > > +		    tx_pkt_len >= (sizeof(struct rte_ether_hdr) + 20 + 8 + 16))
> > > +			tx_pkt_nb_min_segs = i + 1;
> > >  	}
> > > -	if (tx_pkt_len < (sizeof(struct rte_ether_hdr) + 20 + 8)) {
> > > +
> > > +	if (!tx_pkt_nb_min_segs) {
> > >  		fprintf(stderr, "total packet length=%u < %d - give up\n",
> > > -				(unsigned) tx_pkt_len,
> > > -				(int)(sizeof(struct rte_ether_hdr) + 20 + 8));
> > > +			(unsigned int) tx_pkt_len,
> > > +			(int)(sizeof(struct rte_ether_hdr) + 20 + 8 + 16));
> > >  		return;
> > >  	}
> > >
> > > diff --git a/app/test-pmd/testpmd.c b/app/test-pmd/testpmd.c index
> > > 6cbe9ba..c496e59 100644
> > > --- a/app/test-pmd/testpmd.c
> > > +++ b/app/test-pmd/testpmd.c
> > > @@ -232,6 +232,11 @@ struct fwd_engine * fwd_engines[] = {  };
> > > uint8_t tx_pkt_nb_segs = 1; /**< Number of segments in TXONLY
> > > packets */
> > >
> > > +/**< Minimum number of segments in TXONLY packets to accommodate
> > > +all packet
> > > + * headers.
> > > + */
> > > +uint8_t  tx_pkt_nb_min_segs = 1;
> > > +
> > >  enum tx_pkt_split tx_pkt_split = TX_PKT_SPLIT_OFF;  /**< Split
> > > policy for packets to TX. */
> > >
> > > diff --git a/app/test-pmd/testpmd.h b/app/test-pmd/testpmd.h index
> > > 16a3598..f5bc427 100644
> > > --- a/app/test-pmd/testpmd.h
> > > +++ b/app/test-pmd/testpmd.h
> > > @@ -464,6 +464,11 @@ enum dcb_mode_enable  extern uint16_t
> > > tx_pkt_length; /**< Length of TXONLY packet */  extern uint16_t
> > > tx_pkt_seg_lengths[RTE_MAX_SEGS_PER_PKT]; /**< Seg. lengths */
> > > extern uint8_t  tx_pkt_nb_segs; /**< Number of segments in TX
> > > packets */
> > > +
> > > +/**< Minimum number of segments in TXONLY packets to accommodate
> > > +all packet
> > > + * headers.
> > > + */
> > > +extern uint8_t  tx_pkt_nb_min_segs;
> > >  extern uint32_t tx_pkt_times_intra;  extern uint32_t
> > > tx_pkt_times_inter;
> > >
> > > diff --git a/app/test-pmd/txonly.c b/app/test-pmd/txonly.c index
> > > aed820f..27e4458 100644
> > > --- a/app/test-pmd/txonly.c
> > > +++ b/app/test-pmd/txonly.c
> > > @@ -195,8 +195,11 @@
> > >  	uint32_t nb_segs, pkt_len;
> > >  	uint8_t i;
> > >
> > > -	if (unlikely(tx_pkt_split == TX_PKT_SPLIT_RND))
> > > -		nb_segs = rte_rand() % tx_pkt_nb_segs + 1;
> > > +	if (unlikely(tx_pkt_split == TX_PKT_SPLIT_RND) &&
> > > +	    tx_pkt_nb_segs > tx_pkt_nb_min_segs)
> > > +		nb_segs = rte_rand() %
> > > +			  (tx_pkt_nb_segs - tx_pkt_nb_min_segs + 1) +
> > > +			  tx_pkt_nb_min_segs;
> > >  	else
> > >  		nb_segs = tx_pkt_nb_segs;
> > >
> > > --
> > > 1.8.3.1