From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id 2DD30A034E; Thu, 6 Jan 2022 11:51:01 +0100 (CET) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id A7F6641143; Thu, 6 Jan 2022 11:51:00 +0100 (CET) Received: from smartserver.smartsharesystems.com (smartserver.smartsharesystems.com [77.243.40.215]) by mails.dpdk.org (Postfix) with ESMTP id D254B41142 for ; Thu, 6 Jan 2022 11:50:58 +0100 (CET) X-MimeOLE: Produced By Microsoft Exchange V6.5 Content-class: urn:content-classes:message MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Subject: RE: mbuf headroom question Date: Thu, 6 Jan 2022 11:50:54 +0100 Message-ID: <98CBD80474FA8B44BF855DF32C47DC35D86DE2@smartserver.smartshare.dk> In-Reply-To: X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: mbuf headroom question Thread-Index: AdgC4pr0MR56cA82QYe9lPSMKfQO5gABbBWQ References: <98CBD80474FA8B44BF855DF32C47DC35D86DE0@smartserver.smartshare.dk> From: =?iso-8859-1?Q?Morten_Br=F8rup?= To: "Olivier Matz" Cc: X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org > From: Olivier Matz [mailto:olivier.matz@6wind.com] > Sent: Thursday, 6 January 2022 10.49 Thank you for the thorough explanation, Olivier. Somewhat exotic scenarios, but they do make sense! As you might have guessed, I was wondering if = rte_pktmbuf_reset_headroom() could be optimized by simply using = RTE_PKTMBUF_HEADROOM. I still think that it might, but I realize that it = would have wider reaching consequences... >=20 > Hi Morten, >=20 > On Thu, Jan 06, 2022 at 10:29:11AM +0100, Morten Br=F8rup wrote: > > Hi Olivier, > > > > The data_room_size parameter description for the mbuf pool creation > functions says: > > "Size of data buffer in each mbuf, including RTE_PKTMBUF_HEADROOM." > > > > Furthermore, both rte_mbuf_data_iova_default() and > rte_mbuf_data_addr_default() simply add RTE_PKTMBUF_HEADROOM to the > return value. > > > > Based on the above, I would think that it is impossible for m- > >buf_len to be smaller than RTE_PKTMBUF_HEADROOM. > > > > So why does rte_pktmbuf_reset_headroom() use RTE_MIN(m->buf_len, > RTE_PKTMBUF_HEADROOM), instead of just RTE_PKTMBUF_HEADROOM? What am I > missing here? >=20 > It is legal to create a packet pool that has no data buffer: this pool > can be used to allocate packets clones that will be attached to mbufs > containing data. There is an example in test_mbuf.c. In this case, m->data_off is unused, and could be RTE_PKTMBUF_HEADROOM = without causing problems. >=20 > It is also technically possible to create a packet pool with small > mbufs (whose buffer length is less than RTE_PKTMBUF_HEADROOM). These > mbufs cannot be used by drivers which use = rte_mbuf_data_iova_default(), > but they could be used internally. In this case, all of the mbuf's data buffer would be headroom, so the = internal use be application/drivers would need to ignore m->data_ off = anyway, and could be RTE_PKTMBUF_HEADROOM without causing problems. >=20 > To create valid mbufs in these 2 cases, this is why = RTE_MIN(m->buf_len, > RTE_PKTMBUF_HEADROOM) is used ; "valid" means that headroom is not > larger > than buffer length. Validity is important! So if we optimized rte_pktmbuf_reset_headroom(), = all the related validation functions would need to be updated = accordingly. And the description of the data_off field in the mbuf. It is probably not worth the effort pursuing this idea any further. :-) >=20 >=20 > Olivier