> -----Original Message----- > From: Morten Brørup <m...@smartsharesystems.com> > Sent: den 23 juni 2022 09:01 > To: Emil Berg <emil.b...@ericsson.com>; Bruce Richardson > <bruce.richard...@intel.com> > Cc: Stephen Hemminger <step...@networkplumber.org>; > sta...@dpdk.org; bugzi...@dpdk.org; hof...@lysator.liu.se; > olivier.m...@6wind.com; dev@dpdk.org > Subject: RE: [PATCH] net: fix checksum with unaligned buffer > > > From: Emil Berg [mailto:emil.b...@ericsson.com] > > Sent: Thursday, 23 June 2022 07.22 > > > > > From: Morten Brørup <m...@smartsharesystems.com> > > > Sent: den 22 juni 2022 16:02 > > > > > > > From: Emil Berg [mailto:emil.b...@ericsson.com] > > > > Sent: Wednesday, 22 June 2022 14.25 > > > > > > > > > From: Morten Brørup <m...@smartsharesystems.com> > > > > > Sent: den 22 juni 2022 13:26 > > > > > > > > > > > From: Bruce Richardson [mailto:bruce.richard...@intel.com] > > > > > > Sent: Wednesday, 22 June 2022 11.18 > > > > > > > > > > > > On Wed, Jun 22, 2022 at 06:26:07AM +0000, Emil Berg wrote: > > > > > > > > > > > > > > > From: Morten Brørup <m...@smartsharesystems.com> > > > > > > > > Sent: den 21 juni 2022 11:35 > > > > > > > > > > > > > > > > > From: Bruce Richardson > > [mailto:bruce.richard...@intel.com] > > > > > > > > > Sent: Tuesday, 21 June 2022 10.23 > > > > > > > > > > > > > > > > > > On Tue, Jun 21, 2022 at 10:05:15AM +0200, Morten Brørup > > > > 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.b...@ericsson.com] > > > > > > > > > > > Sent: Tuesday, 21 June 2022 09.17 > > > > > > > > > > > > > > > > > > > > > > > From: Morten Brørup <m...@smartsharesystems.com> > > > > > > > > > > > > Sent: den 20 juni 2022 12:58 > > > > > > > > > > > > > > > > > > > > > > > > > From: Emil Berg [mailto:emil.b...@ericsson.com] > > > > > > > > > > > > > Sent: Monday, 20 June 2022 12.38 > > > > > > > > > > > > > > > > > > > > > > > > > > > From: Morten Brørup > <m...@smartsharesystems.com> > > > > > > > > > > > > > > Sent: den 17 juni 2022 11:07 > > > > > > > > > > > > > > > > > > > > > > > > > > > > > From: Morten Brørup > > > > > > > > > > > > > > > [mailto:m...@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: sta...@dpdk.org > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Signed-off-by: Morten Brørup > > > > > > <m...@smartsharesystems.com> > > > > > > > > > > > > > > > --- > > > > > > > > > > > > > > > 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 = (const u16_p > > *)buf; > > > > > > > > > > > > > > > - const u16_p *end = 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 = 0; > > > > > > > > > > > > > > > + if (unlikely(len == 0)) > > > > > > > > > > > > > > > + return 0; > > > > > > > > > > > > > > > + ((unsigned char *)&first)[1] = > > > > > *(const > > > > > > unsigned > > > > > > > > > > > > > > char *)buf; > > > > > > > > > > > > > > > + sum += first; > > > > > > > > > > > > > > > + buf = (const void *)((uintptr_t)buf > > > > > + 1); > > > > > > > > > > > > > > > + len--; > > > > > > > > > > > > > > > + } > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > + u16_buf = (const u16_p *)buf; > > > > > > > > > > > > > > > + end = u16_buf + len / sizeof(*u16_buf); > > > > > > > > > > > > > > > for (; u16_buf != end; ++u16_buf) > > > > > > > > > > > > > > > sum += *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 > > > > > > > > > > > > > Rönnblom 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. > > > > > > > > > > > > > > > > > > > If PDCP headers can leave the rest of the packet field > > unaligned, > > > > then > > > > > > we had better remove the alignment restrictions through all of > > > > DPDK. > > > > > > > > > > > > /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://protect2.fireeye.com/v1/url?k=31323334-501cfaf3-313273af > > > > > - > > > > > 454445554331-2ffe58e5caaeb74e&q=1&e=3f0544d3-8a71-4676-b4f9- > > > > > > > > > 27e0952f7de0&u=https%3A%2F%2Fbugs.dpdk.org%2Fshow_bug.cgi%3Fid% > > > > > 3D1035 > > > > > > > > That's my interpretation of the standard as well; For example an > > > > uint16_t* must be on even addresses. If not it is undefined > > behavior. > > > > I think this is a bigger problem on ARM for example. > > > > > > > > Without being that invested in dpdk, adding unaligned support for > > > > everything seems like a steep step, but I'm not sure what it > > entails > > > > in practice. > > > > > > > > We are actually only interested in the checksumming. > > > > > > Great! Then we can cancel the panic about rewriting DPDK Core > > completely. > > > Although it might still need some review for similar alignment bugs, > > where > > > we have been forcing the compiler shut up when trying to warn us. > > > :-) > > > > > > I have provided v3 of the patch, which should do as requested - and > > still allow > > > the compiler to auto-vectorize. > > > > > > @Emil, will you please test v3 of the patch? > > > > It seems to work in these two cases: > > * Even address, even length > > * Even address, odd length > > But it breaks in these two cases: > > * Odd address, even length (although it works for small buffers, > > probably when the sum fits inside a uint16_t integer or something) > > Interesting observation, good analysis. > > > * Odd address, odd length > > Does this also work for small buffers? > > > I get (and like) the main idea of the algorithm but haven't yet > > figured out what the problem is with odd addresses. > > I wonder if I messed up the algorithm for swapping back the bytes in bsum > after the calculation... Is the checksum also wrong when compiling without > optimization? > > And just to be sure: The algorithm requires that __rte_raw_cksum_reduce() > is also applied to the sum. Please confirm that you call rte_raw_cksum() (or > __rte_raw_cksum() followed by __rte_raw_cksum_reduce())? > Yes, I messed up. I didn't run the reduction part. When I do the output seems to be the same. It seems to be about as fast as the previous algorithm, obviously. Both valgrind and fsanitize=undefined are happy. Some minor improvements: * #include <stdbool.h>? * Use RTE_PTR_ADD to make the casts cleaner? * I guess you could skip using 'bsum' and add to 'sum' instead, but that's a matter of preference * Can't you just do bsum += *(const unsigned char *)buf; to avoid 'first', making it a bit more readable? > > > > /Emil
