Re: [dpdk-dev] [PATCH v5 1/2] mbuf: support attaching external buffer to mbuf

[Yongseok Koh <yskoh@mellanox.com>]

On Wed, Apr 25, 2018 at 01:31:42PM +0000, Ananyev, Konstantin wrote:
[...]
> >  /** Mbuf prefetch */
> >  #define RTE_MBUF_PREFETCH_TO_FREE(m) do {       \
> >  	if ((m) != NULL)                        \
> > @@ -1213,11 +1306,127 @@ static inline int rte_pktmbuf_alloc_bulk(struct rte_mempool *pool,
> >  }
> > 
> >  /**
> > + * Attach an external buffer to a mbuf.
> > + *
> > + * User-managed anonymous buffer can be attached to an mbuf. When attaching
> > + * it, corresponding free callback function and its argument should be
> > + * provided. This callback function will be called once all the mbufs are
> > + * detached from the buffer.
> > + *
> > + * The headroom for the attaching mbuf will be set to zero and this can be
> > + * properly adjusted after attachment. For example, ``rte_pktmbuf_adj()``
> > + * or ``rte_pktmbuf_reset_headroom()`` can be used.
> > + *
> > + * More mbufs can be attached to the same external buffer by
> > + * ``rte_pktmbuf_attach()`` once the external buffer has been attached by
> > + * this API.
> > + *
> > + * Detachment can be done by either ``rte_pktmbuf_detach_extbuf()`` or
> > + * ``rte_pktmbuf_detach()``.
> > + *
> > + * Attaching an external buffer is quite similar to mbuf indirection in
> > + * replacing buffer addresses and length of a mbuf, but a few differences:
> > + * - When an indirect mbuf is attached, refcnt of the direct mbuf would be
> > + *   2 as long as the direct mbuf itself isn't freed after the attachment.
> > + *   In such cases, the buffer area of a direct mbuf must be read-only. But
> > + *   external buffer has its own refcnt and it starts from 1. Unless
> > + *   multiple mbufs are attached to a mbuf having an external buffer, the
> > + *   external buffer is writable.
> > + * - There's no need to allocate buffer from a mempool. Any buffer can be
> > + *   attached with appropriate free callback and its IO address.
> > + * - Smaller metadata is required to maintain shared data such as refcnt.
> > + *
> > + * @warning
> > + * @b EXPERIMENTAL: This API may change without prior notice.
> > + * Once external buffer is enabled by allowing experimental API,
> > + * ``RTE_MBUF_DIRECT()`` and ``RTE_MBUF_INDIRECT()`` are no longer
> > + * exclusive. A mbuf can be considered direct if it is neither indirect nor
> > + * having external buffer.
> > + *
> > + * @param m
> > + *   The pointer to the mbuf.
> > + * @param buf_addr
> > + *   The pointer to the external buffer we're attaching to.
> > + * @param buf_iova
> > + *   IO address of the external buffer we're attaching to.
> > + * @param buf_len
> > + *   The size of the external buffer we're attaching to. If memory for
> > + *   shared data is not provided, buf_len must be larger than the size of
> > + *   ``struct rte_mbuf_ext_shared_info`` and padding for alignment. If not
> > + *   enough, this function will return NULL.
> > + * @param shinfo
> > + *   User-provided memory for shared data. If NULL, a few bytes in the
> > + *   trailer of the provided buffer will be dedicated for shared data and
> > + *   the shared data will be properly initialized. Otherwise, user must
> > + *   initialize the content except for free callback and its argument. The
> > + *   pointer of shared data will be stored in m->shinfo.
> > + * @param free_cb
> > + *   Free callback function to call when the external buffer needs to be
> > + *   freed.
> > + * @param fcb_opaque
> > + *   Argument for the free callback function.
> > + *
> > + * @return
> > + *   A pointer to the new start of the data on success, return NULL
> > + *   otherwise.
> > + */
> > +static inline char * __rte_experimental
> > +rte_pktmbuf_attach_extbuf(struct rte_mbuf *m, void *buf_addr,
> > +	rte_iova_t buf_iova, uint16_t buf_len,
> > +	struct rte_mbuf_ext_shared_info *shinfo,
> > +	rte_mbuf_extbuf_free_callback_t free_cb, void *fcb_opaque)
> > +{
> > +	/* Additional attachment should be done by rte_pktmbuf_attach() */
> > +	RTE_ASSERT(!RTE_MBUF_HAS_EXTBUF(m));
> 
> Shouldn't we have here something like:
> RTE_ASSERT(RTE_MBUF_DIRECT(m) && rte_mbuf_refcnt_read(m) == 1);
> ?

Right. That's better. Attaching mbuf should be direct and writable.

> > +
> > +	m->buf_addr = buf_addr;
> > +	m->buf_iova = buf_iova;
> > +
> > +	if (shinfo == NULL) {
> 
> Instead of allocating shinfo ourselves - wound's it be better to rely
> on caller always allocating afeeling it for us (he can do that at the end/start of buffer,
> or whenever he likes to.

It is just for convenience. For some users, external attachment could be
occasional and casual, e.g. punt control traffic from kernel/hv. For such
non-serious cases, it is good to provide this small utility.

> Again in that case - caller can provide one shinfo to several mbufs (with different buf_addrs)
> and would know for sure that free_cb wouldn't be overwritten by mistake.
> I.E. mbuf code will only update refcnt inside shinfo.

I think you missed the discussion with other people yesterday. This change is
exactly for that purpose. Like I documented above, if this API is called with
shinfo being provided, it will use the user-provided shinfo instead of sparing a
few byte in the trailer and won't touch the shinfo. This code block happens only
if user doesn't provide memory for shared data (shinfo is NULL).

> > +		void *buf_end = RTE_PTR_ADD(buf_addr, buf_len);
> > +
> > +		shinfo = RTE_PTR_ALIGN_FLOOR(RTE_PTR_SUB(buf_end,
> > +					sizeof(*shinfo)), sizeof(uintptr_t));
> > +		if ((void *)shinfo <= buf_addr)
> > +			return NULL;
> > +
> > +		m->buf_len = RTE_PTR_DIFF(shinfo, buf_addr);
> > +		rte_mbuf_ext_refcnt_set(shinfo, 1);
> > +	} else {
> > +		m->buf_len = buf_len;
> 
> I think you need to update shinfo>refcnt here too.

Like explained above, if shinfo is provided, it doesn't alter anything except
for callbacks and its arg.


Thanks,
Yongseok