Re: [dpdk-dev] [PATCH v2] ixgbe: don't override mbuf buffer length

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Jean-Mickael Guerin [mailto:jean-mickael.guerin@6wind.com]
> Sent: Thursday, December 04, 2014 6:09 PM
> To: dev@dpdk.org
> Cc: Richardson, Bruce; Ananyev, Konstantin
> Subject: [PATCH v2] ixgbe: don't override mbuf buffer length
> 
> The template mbuf_initializer is hard coded with a buflen which
> might have been set differently by the application at the time of
> mbuf pool creation.
> 
> Switch to a mbuf allocation, to fetch the correct default values.
> There is no performance impact because this is not a data-plane API.
> 
> Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>
> Acked-by: David Marchand <david.marchand@6wind.com>
> Fixes: 0ff3324da2 ("ixgbe: rework vector pmd following mbuf changes")
> ---
> 
>  v2: check returned value of ixgbe_rxq_vec_setup
> 
>  lib/librte_pmd_ixgbe/ixgbe_rxtx.c     |  5 ++++-
>  lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c | 19 ++++++++++++-------
>  2 files changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> index 5c36bff..7994da1 100644
> --- a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> +++ b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> @@ -2244,7 +2244,10 @@ ixgbe_dev_rx_queue_setup(struct rte_eth_dev *dev,
>  	use_def_burst_func = check_rx_burst_bulk_alloc_preconditions(rxq);
> 
>  #ifdef RTE_IXGBE_INC_VECTOR
> -	ixgbe_rxq_vec_setup(rxq);
> +	if (ixgbe_rxq_vec_setup(rxq) < 0) {
> +		ixgbe_rx_queue_release(rxq);
> +		return (-ENOMEM);
> +	}
>  #endif
>  	/* Check if pre-conditions are satisfied, and no Scattered Rx */
>  	if (!use_def_burst_func && !dev->data->scattered_rx) {
> diff --git a/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c b/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
> index c1b5a78..f7b02f5 100644
> --- a/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
> +++ b/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
> @@ -732,17 +732,22 @@ static struct ixgbe_txq_ops vec_txq_ops = {
>  int
>  ixgbe_rxq_vec_setup(struct igb_rx_queue *rxq)
>  {
> -	struct rte_mbuf mb_def = { .buf_addr = 0 }; /* zeroed mbuf */
> +	struct rte_mbuf *mb_def;
> 
> -	mb_def.nb_segs = 1;
> -	mb_def.data_off = RTE_PKTMBUF_HEADROOM;
> -	mb_def.buf_len = rxq->mb_pool->elt_size - sizeof(struct rte_mbuf);
> -	mb_def.port = rxq->port_id;
> -	rte_mbuf_refcnt_set(&mb_def, 1);
> +	mb_def = rte_pktmbuf_alloc(rxq->mb_pool);
> +	if (mb_def == NULL) {
> +		PMD_INIT_LOG(ERR, "ixgbe_rxq_vec_setup: could not allocate one mbuf");
> +		return -1;
> +	}
> +	/* nb_segs, refcnt, data_off and buf_len are already set */
> +	mb_def->port = rxq->port_id;
> 
>  	/* prevent compiler reordering: rearm_data covers previous fields */
>  	rte_compiler_barrier();
> -	rxq->mbuf_initializer = *((uint64_t *)&mb_def.rearm_data);
> +	rxq->mbuf_initializer = *((uint64_t *)&mb_def->rearm_data);
> +
> +	rte_pktmbuf_free(mb_def);
> +
>  	return 0;
>  }
> 
> --
> 2.1.3

As I said in another mail, I don't think it is a proper fix.
What we did here - just changed one assumption to another.
Current assumption - all mbuf obj_init() would setup buf_len in exactly the same manner as  rte_pktmbuf_init() does:
buf_len = mp->elt_size - sizeof(struct rte_mbuf);
New assumption - all mbuf obj_init() would setup buf_len for all mbufs in the pool to the same value.
Both assumptions, I believe, are not always correct.
Though, probably the new one would be true more often.

I still think the proper fix is not to update mbuf's buf_len field at ixgbe_rxq_rearm() at all.
We should just leave the original value unmodified.
Actually, while looking at ixgbe_rxq_rearm(), I don't see any reason why we need to update buf_len field.
It is not the data that need to be rearmed.
The fields that need to be rearmed are:
uint16_t data_off;
uint16_t refcnt
uint8_t nb_segs;
uint8_t port;

6B in total. 
We probably would like to keep rearming as one 64bit load/store. 
Though straight below them we have:
uint64_t ol_flags;

As RX fully override ol_flags anyway, we can safely overwrite first 2B of it.
That would allow us to still read/write whole 64bits and avoid overwriting buf_len.  
I am talking about something like patch  below.
I admit that it looks not so pretty, but I think it is much safer and correct.
Konstantin

--- a/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
+++ b/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
@@ -79,13 +79,19 @@ ixgbe_rxq_rearm(struct igb_rx_queue *rxq)
        /* Initialize the mbufs in vector, process 2 mbufs in one loop */
        for (i = 0; i < RTE_IXGBE_RXQ_REARM_THRESH; i += 2, rxep += 2) {
                __m128i vaddr0, vaddr1;
+               uintptr_t p0, p1;

                mb0 = rxep[0].mbuf;
                mb1 = rxep[1].mbuf;

                /* flush mbuf with pkt template */
-               mb0->rearm_data[0] = rxq->mbuf_initializer;
-               mb1->rearm_data[0] = rxq->mbuf_initializer;
+               p0 = (uintptr_t)&mb0->data_off;
+               *(uint64_t *)p0 = rxq->mbuf_initializer;
+               p1 = (uintptr_t)&mb1->data_off;
+               *(uint64_t *)p1 = rxq->mbuf_initializer;
+
+               //mb0->rearm_data[0] = rxq->mbuf_initializer;
+               //mb1->rearm_data[0] = rxq->mbuf_initializer;

                /* load buf_addr(lo 64bit) and buf_physaddr(hi 64bit) */
                vaddr0 = _mm_loadu_si128((__m128i *)&(mb0->buf_addr));
@@ -732,6 +738,7 @@ static struct ixgbe_txq_ops vec_txq_ops = {
 int
 ixgbe_rxq_vec_setup(struct igb_rx_queue *rxq)
 {
+       uintptr_t p;
        struct rte_mbuf mb_def = { .buf_addr = 0 }; /* zeroed mbuf */

        mb_def.nb_segs = 1;
@@ -739,7 +746,8 @@ ixgbe_rxq_vec_setup(struct igb_rx_queue *rxq)
        mb_def.buf_len = rxq->mb_pool->elt_size - sizeof(struct rte_mbuf);
        mb_def.port = rxq->port_id;
        rte_mbuf_refcnt_set(&mb_def, 1);
-       rxq->mbuf_initializer = *((uint64_t *)&mb_def.rearm_data);
+       p = (uintptr_t)&mb_def.data_off;
+       rxq->mbuf_initializer = *(uint64_t *)p;
        return 0;
 }