[dpdk-dev] mbuff rearm_data aligmenet issue on non x86

[Ananyev, Konstantin]

Hi Jerrin,

> 
> Hi All,
> 
> I would like align mbuff rearm_data field to 8 byte aligned so that
> write to mbuf->rearm_data with uint64_t* will be naturally aligned.
> I am not sure about IA but some other architecture/implementation has overhead
> in non-naturally aligned stores.
> 
> Proposed patch is something like this below, But open for any change to
> make fit for all other architectures/platform.
> 
> Any thoughts ?
> 
> ➜ [master] [dpdk-master] $ git diff
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index 529debb..5a917d0 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -733,10 +733,8 @@ struct rte_mbuf {
>         void *buf_addr;           /**< Virtual address of segment
> buffer. */
>         phys_addr_t buf_physaddr; /**< Physical address of segment
> buffer. */
> 
> -       uint16_t buf_len;         /**< Length of segment buffer. */
> -


There is no need to move buf_len itself, I think.
Just move rearm_data marker prior to buf_len is enough.
Though how do you suggest to deal with the fact, that right now we blindly
update the whole 64bits pointed by rearm_data:

drivers/net/ixgbe/ixgbe_rxtx_vec.c:
	/*
                 * Flush mbuf with pkt template.
                 * Data to be rearmed is 6 bytes long.
                 * Though, RX will overwrite ol_flags that are coming next
                 * anyway. So overwrite whole 8 bytes with one load:
                 * 6 bytes of rearm_data plus first 2 bytes of ol_flags.
                 */
                p0 = (uintptr_t)&mb0->rearm_data;
                *(uint64_t *)p0 = rxq->mbuf_initializer;

?

If buf_len will be inside these 64bits, we can't do it anymore.

Are you suggesting something like:

uint64_t *p0, v0; 

p0 = &mb0->rearm_data;
v0 = *p0 & REARM_MASK;
*p0 = v0 | rxq->mbuf_initializer;
? 

If so I wonder what would be the performance impact of that change.
Konstantin


>         /* next 6 bytes are initialised on RX descriptor rearm */
> -       MARKER8 rearm_data;
> +       MARKER64 rearm_data;
>         uint16_t data_off;
> 
>         /**
> @@ -754,6 +752,7 @@ struct rte_mbuf {
>         uint8_t nb_segs;          /**< Number of segments. */
>         uint8_t port;             /**< Input port. */
> 
> +       uint16_t buf_len;         /**< Length of segment buffer. */
>         uint64_t ol_flags;        /**< Offload features. */
> 
>         /* remaining bytes are set on RX when pulling packet from
>  * descriptor
> 
> /Jerin