From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by smtp.lore.kernel.org (Postfix) with ESMTP id C6FB9C433EF for ; Wed, 22 Jun 2022 11:26:12 +0000 (UTC) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 00FBB4069C; Wed, 22 Jun 2022 13:26:12 +0200 (CEST) Received: from smartserver.smartsharesystems.com (smartserver.smartsharesystems.com [77.243.40.215]) by mails.dpdk.org (Postfix) with ESMTP id BCDE340689; Wed, 22 Jun 2022 13:26:09 +0200 (CEST) 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: [PATCH] net: fix checksum with unaligned buffer X-MimeOLE: Produced By Microsoft Exchange V6.5 Date: Wed, 22 Jun 2022 13:26:05 +0200 Message-ID: <98CBD80474FA8B44BF855DF32C47DC35D87152@smartserver.smartshare.dk> In-Reply-To: X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: [PATCH] net: fix checksum with unaligned buffer Thread-Index: AdiGGQg4o5cEIFZiQHOSyoekKAnYfgAC2L+A References: <98CBD80474FA8B44BF855DF32C47DC35D87139@smartserver.smartshare.dk> <20220617084505.62071-1-mb@smartsharesystems.com> <98CBD80474FA8B44BF855DF32C47DC35D8713A@smartserver.smartshare.dk> <98CBD80474FA8B44BF855DF32C47DC35D87141@smartserver.smartshare.dk> <98CBD80474FA8B44BF855DF32C47DC35D87145@smartserver.smartshare.dk> <98CBD80474FA8B44BF855DF32C47DC35D87148@smartserver.smartshare.dk> From: =?iso-8859-1?Q?Morten_Br=F8rup?= To: "Bruce Richardson" , "Emil Berg" Cc: "Stephen Hemminger" , , , , , 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: Bruce Richardson [mailto:bruce.richardson@intel.com] > Sent: Wednesday, 22 June 2022 11.18 >=20 > On Wed, Jun 22, 2022 at 06:26:07AM +0000, Emil Berg wrote: > > > > > From: Morten Br=F8rup > > > Sent: den 21 juni 2022 11:35 > > > > > > > From: Bruce Richardson [mailto:bruce.richardson@intel.com] > > > > Sent: Tuesday, 21 June 2022 10.23 > > > > > > > > On Tue, Jun 21, 2022 at 10:05:15AM +0200, Morten Br=F8rup wrote: > > > > > +TO: @Bruce and @Stephen: You signed off on the 16 bit > alignment > > > > requirement. We need background info on this. > > > > > > > > > > > From: Emil Berg [mailto:emil.berg@ericsson.com] > > > > > > Sent: Tuesday, 21 June 2022 09.17 > > > > > > > > > > > > > From: Morten Br=F8rup > > > > > > > Sent: den 20 juni 2022 12:58 > > > > > > > > > > > > > > > From: Emil Berg [mailto:emil.berg@ericsson.com] > > > > > > > > Sent: Monday, 20 June 2022 12.38 > > > > > > > > > > > > > > > > > From: Morten Br=F8rup > > > > > > > > > Sent: den 17 juni 2022 11:07 > > > > > > > > > > > > > > > > > > > From: Morten Br=F8rup = [mailto:mb@smartsharesystems.com] > > > > > > > > > > Sent: Friday, 17 June 2022 10.45 > > > > > > > > > > > > > > > > > > > > With this patch, the checksum can be calculated on = an > > > > unligned > > > > > > > > > > part > > > > > > > > of > > > > > > > > > > a packet buffer. > > > > > > > > > > I.e. the buf parameter is no longer required to be = 16 > bit > > > > > > aligned. > > > > > > > > > > > > > > > > > > > > The DPDK invariant that packet buffers must be 16 = bit > > > > aligned > > > > > > > > remains > > > > > > > > > > unchanged. > > > > > > > > > > This invariant also defines how to calculate the 16 > bit > > > > > > checksum > > > > > > > > > > on > > > > > > > > an > > > > > > > > > > unaligned part of a packet buffer. > > > > > > > > > > > > > > > > > > > > Bugzilla ID: 1035 > > > > > > > > > > Cc: stable@dpdk.org > > > > > > > > > > > > > > > > > > > > Signed-off-by: Morten Br=F8rup > > > > > > > > > > > --- > > > > > > > > > > lib/net/rte_ip.h | 17 +++++++++++++++-- > > > > > > > > > > 1 file changed, 15 insertions(+), 2 deletions(-) > > > > > > > > > > > > > > > > > > > > diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h > index > > > > > > > > > > b502481670..8e301d9c26 100644 > > > > > > > > > > --- a/lib/net/rte_ip.h > > > > > > > > > > +++ b/lib/net/rte_ip.h > > > > > > > > > > @@ -162,9 +162,22 @@ __rte_raw_cksum(const void = *buf, > > > > size_t > > > > > > len, > > > > > > > > > > uint32_t sum) { > > > > > > > > > > /* extend strict-aliasing rules */ > > > > > > > > > > typedef uint16_t __attribute__((__may_alias__)) > > > > u16_p; > > > > > > > > > > - const u16_p *u16_buf =3D (const u16_p *)buf; > > > > > > > > > > - const u16_p *end =3D u16_buf + len / > sizeof(*u16_buf); > > > > > > > > > > + const u16_p *u16_buf; > > > > > > > > > > + const u16_p *end; > > > > > > > > > > + > > > > > > > > > > + /* if buffer is unaligned, keeping it byte > order > > > > > > independent */ > > > > > > > > > > + if (unlikely((uintptr_t)buf & 1)) { > > > > > > > > > > + uint16_t first =3D 0; > > > > > > > > > > + if (unlikely(len =3D=3D 0)) > > > > > > > > > > + return 0; > > > > > > > > > > + ((unsigned char *)&first)[1] =3D *(const > unsigned > > > > > > > > > char *)buf; > > > > > > > > > > + sum +=3D first; > > > > > > > > > > + buf =3D (const void *)((uintptr_t)buf + 1); > > > > > > > > > > + len--; > > > > > > > > > > + } > > > > > > > > > > > > > > > > > > > > + u16_buf =3D (const u16_p *)buf; > > > > > > > > > > + end =3D u16_buf + len / sizeof(*u16_buf); > > > > > > > > > > for (; u16_buf !=3D end; ++u16_buf) > > > > > > > > > > sum +=3D *u16_buf; > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > 2.17.1 > > > > > > > > > > > > > > > > > > @Emil, can you please test this patch with an = unaligned > > > > buffer on > > > > > > > > your > > > > > > > > > application to confirm that it produces the expected > result. > > > > > > > > > > > > > > > > Hi! > > > > > > > > > > > > > > > > I tested the patch. It doesn't seem to produce the same > > > > results. I > > > > > > > > think the problem is that it always starts summing from > an > > > > > > > > even address, the sum should always start from the first > byte > > > > according > > > > > > to > > > > > > > > the checksum specification. Can I instead propose > something > > > > Mattias > > > > > > > > R=F6nnblom sent me? > > > > > > > > > > > > > > I assume that it produces the same result when the "buf" > > > > parameter is > > > > > > > aligned? > > > > > > > > > > > > > > And when the "buf" parameter is unaligned, I don't expect > it to > > > > > > produce the > > > > > > > same results as the simple algorithm! > > > > > > > > > > > > > > This was the whole point of the patch: I expect the = overall > > > > packet > > > > > > buffer to > > > > > > > be 16 bit aligned, and the checksum to be a partial > checksum of > > > > such > > > > > > a 16 bit > > > > > > > aligned packet buffer. When calling this function, I = assume > that > > > > the > > > > > > "buf" and > > > > > > > "len" parameters point to a part of such a packet buffer. > If > > > > these > > > > > > > expectations are correct, the simple algorithm will = produce > > > > incorrect > > > > > > results > > > > > > > when "buf" is unaligned. > > > > > > > > > > > > > > I was asking you to test if the checksum on the packet is > > > > > > > correct > > > > > > when your > > > > > > > application modifies an unaligned part of the packet and > uses > > > > this > > > > > > function to > > > > > > > update the checksum. > > > > > > > > > > > > > > > > > > > Now I understand your use case. Your use case seems to be > about > > > > partial > > > > > > checksums, of which some partial checksums may start on > unaligned > > > > > > addresses in an otherwise aligned packet. > > > > > > > > > > > > Our use case is about calculating the full checksum on a > nested > > > > packet. > > > > > > That nested packet may start on unaligned addresses. > > > > > > > > > > > > The difference is basically if we want to sum over aligned > > > > addresses or > > > > > > not, handling the heading and trailing bytes appropriately. > > > > > > > > > > > > Your method does not work in our case since we want to treat > the > > > > first > > > > > > two bytes as the first word in our case. But I do understand > that > > > > both > > > > > > methods are useful. > > > > > > > > > > Yes, that certainly are two different use cases, requiring two > > > > different ways of calculating the 16 bit checksum. > > > > > > > > > > > > > > > > > Note that your method breaks the API. Previously (assuming = no > > > > crashing > > > > > > due to low optimization levels, more accepting hardware, or = a > > > > different > > > > > > compiler (version)) the current method would calculate the > > > > > > checksum assuming the first two bytes is the first word. > > > > > > > > > > > > > > > > Depending on the point of view, my patch either fixes a bug > (where > > > > the checksum was calculated incorrectly when the buf pointer was > > > > unaligned) or breaks the API (by calculating the differently = when > the > > > > buffer is unaligned). > > > > > > > > > > I cannot say with certainty which one is correct, but perhaps > some > > > > > of > > > > the people with a deeper DPDK track record can... > > > > > > > > > > @Bruce and @Stephen, in 2019 you signed off on a patch [1] > > > > introducing a 16 bit alignment requirement to the Ethernet > address > > > > structure. > > > > > > > > > > It is my understanding that DPDK has an invariant requiring > packets > > > > to be 16 bit aligned, which that patch supports. Is this > invariant > > > > documented anywhere, or am I completely wrong? If I'm wrong, = then > the > > > > alignment requirement introduced in that patch needs to be > removed, as > > > > well as any similar alignment requirements elsewhere in DPDK. > > > > > > > > I don't believe it is explicitly documented as a global > invariant, but > > > > I think it should be unless there is a definite case where we > need to > > > > allow packets to be completely unaligned. Across all packet > headers we > > > > looked at, there was no tunneling protocol where the resulting > packet > > > > was left unaligned. > > > > > > > > That said, if there are real use cases where we need to allow > packets > > > > to start at an unaligned address, then I agree with you that we > need > > > > to roll back the patch and work to ensure everything works with > > > > unaligned addresses. > > > > > > > > /Bruce > > > > > > > > > > @Emil, can you please describe or refer to which tunneling = protocol > you are > > > using, where the nested packet can be unaligned? > > > > > > I am asking to determine if your use case is exotic (maybe some > Ericsson > > > proprietary protocol), or more generic (rooted in some standard > protocol). > > > This information affects the DPDK community's opinion about how it > should > > > be supported by DPDK. > > > > > > If possible, please provide more details about the tunneling > protocol and > > > nested packets... E.g. do the nested packets also contain Layer 2 > (Ethernet, > > > VLAN, etc.) headers, or only Layer 3 (IP) or Layer 4 (TCP, UDP, > etc.)? And how > > > about ARP packets and Layer 2 control protocol packets (STP, LACP, > etc.)? > > > > > > > Well, if you append or adjust an odd number of bytes (e.g. a PDCP > header) from a previously aligned payload the entire packet will then > be unaligned. > > >=20 > If PDCP headers can leave the rest of the packet field unaligned, then > we > had better remove the alignment restrictions through all of DPDK. >=20 > /Bruce Re-reading the details regarding unaligned pointers in C11, as posted by = Emil in Bugzilla [2], I interpret it as follows: Any 16 bit or wider = pointer type a must point to data aligned with that type, i.e. a pointer = of the type "uint16_t *" must point to 16 bit aligned data, and a = pointer of the type "uint64_t *" must point to 64 bit aligned data. = Please, someone tell me I got this wrong, and wake me up from my = nightmare! Updating DPDK's packet structures to fully support this C11 limitation = with unaligned access would be a nightmare, as we would need to use byte = arrays for all structure fields. Functions would also be unable to use = other pointer types than "void *" and "char *", which seems to be the = actual problem in the __rte_raw_cksum() function. I guess that it also = would prevent the compiler from auto-vectorizing the functions. I am usually a big proponent of academically correct solutions, but such = a change would be too wide ranging, so I would like to narrow it down to = the actual use case, and perhaps extrapolate a bit from there. @Emil: Do you only need to calculate the checksum of the (potentially = unaligned) embedded packet? Or do you also need to use other DPDK = functions with the embedded packet, potentially accessing it at an = unaligned address? I'm trying to determine the scope of this C11 pointer alignment = limitation for your use case, i.e. whether or not other DPDK functions = need to be updated to support unaligned packet access too. [2] https://bugs.dpdk.org/show_bug.cgi?id=3D1035