From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (xvm-189-124.dc0.ghst.net [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id 5D284A0524; Thu, 7 Jan 2021 14:06:58 +0100 (CET) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id EA09B140F50; Thu, 7 Jan 2021 14:06:57 +0100 (CET) Received: from mga06.intel.com (mga06.intel.com [134.134.136.31]) by mails.dpdk.org (Postfix) with ESMTP id 2F8F5140F4F for ; Thu, 7 Jan 2021 14:06:56 +0100 (CET) IronPort-SDR: QAHOxIX181l7IVAJrU99nF6ekg1E6rTQH3I+mxTylkbCn71hwes5QrXsJ8gOmzQy1mkclLYH1/ XpV+QBQYNKQw== X-IronPort-AV: E=McAfee;i="6000,8403,9856"; a="238972366" X-IronPort-AV: E=Sophos;i="5.79,329,1602572400"; d="scan'208";a="238972366" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Jan 2021 05:06:54 -0800 IronPort-SDR: t4xbCDvpCfkhkYzOlo0kA+wzmbkQtyp8XhmBA+QfCcPq3R8a9gpOnvzOdHSo+sV1SRO3sKBRoM J5NdX611J3dg== X-IronPort-AV: E=Sophos;i="5.79,329,1602572400"; d="scan'208";a="361960406" Received: from fyigit-mobl1.ger.corp.intel.com (HELO [10.252.29.165]) ([10.252.29.165]) by orsmga002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Jan 2021 05:06:53 -0800 From: Ferruh Yigit To: George Prekas , Wenzhuo Lu , Beilei Xing , Bernard Iremonger Cc: dev@dpdk.org, Stephen Hemminger , Harry van Haaren References: <20201203135954.1127-1-prekageo@amazon.com> <20201205054238.12469-1-prekageo@amazon.com> <8c1fd6ad-302d-f45e-f48a-e81fd5f8ba85@intel.com> <6539f363-97b1-82b2-1b09-036aa75c9dc9@amazon.com> <6a84a356-191a-fca6-607f-3caf88eb3da6@intel.com> Message-ID: Date: Thu, 7 Jan 2021 13:06:49 +0000 MIME-Version: 1.0 In-Reply-To: <6a84a356-191a-fca6-607f-3caf88eb3da6@intel.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Subject: Re: [dpdk-dev] [PATCH v2] app/testpmd: fix IP checksum calculation 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 Sender: "dev" On 1/7/2021 11:32 AM, Ferruh Yigit wrote: > On 1/7/2021 5:39 AM, George Prekas wrote: >> >> >> On 1/6/2021 12:02 PM, Ferruh Yigit wrote: >>> On 12/5/2020 5:42 AM, George Prekas wrote: >>>> Strict-aliasing rules are violated by cast to uint16_t* in flowgen.c >>>> and the calculated IP checksum is wrong on GCC 9 and GCC 10. >>>> >>>> Signed-off-by: George Prekas >>>> --- >>>> v2: >>>> * Instead of a compiler barrier, use a compiler flag. >>>> --- >>>>    app/test-pmd/meson.build | 1 + >>>>    1 file changed, 1 insertion(+) >>>> >>>> diff --git a/app/test-pmd/meson.build b/app/test-pmd/meson.build >>>> index 7e9c7bdd6..5d24e807f 100644 >>>> --- a/app/test-pmd/meson.build >>>> +++ b/app/test-pmd/meson.build >>>> @@ -4,6 +4,7 @@ >>>>    # override default name to drop the hyphen >>>>    name = 'testpmd' >>>>    cflags += '-Wno-deprecated-declarations' >>>> +cflags += '-fno-strict-aliasing' >>>>    sources = files('5tswap.c', >>>>        'cmdline.c', >>>>        'cmdline_flow.c', >>>> >>> >>> Hi George, >>> >>> I am trying to understand this, the relevant code is as below: >>> ip_hdr->hdr_checksum = ip_sum((unaligned_uint16_t *)ip_hdr, sizeof(*ip_hdr)); >>> >>> You are suspicious of strict aliasing rule violation, with more details: >>> The concern is the "struct rte_ipv4_hdr *ip_hdr;" aliased to "const >>> unaligned_uint16_t *hdr", and compiler can optimize out the calculations using >>> data pointed by 'hdr' pointer, since the 'hdr' pointer is not used to alter the >>> data and compiler may think data is not changed at all. >>> >>> 1) But the pointer "hdr" is assigned in the loop, from another pointer whose >>> content is changing, why this is not helping to figure out that the data 'hdr' >>> pointing is changed. >>> >>> 2) I tried to debug this, but I am not able to reproduce the issue, 'ip_sum()' >>> called each time and checksum calculated correctly. Using gcc 10.2.1-9. Can you >>> able to confirm the case with debug, or from the assembly/object file? >>> >>> >>> And if the issue is strict aliasing rule violation as you said, compiler flag is >>> an option but not sure how much it reduces the compiler optimization benefit, I >>> guess other options also not so good, memcpy brings too much work on runtime and >>> union requires bigger change and makes code complex. >>> I wonder if making 'ip_sum()' a non inline function can help, can you please >>> give a try since you can reproduce it? >> >> Hi Ferruh, >> >> Thanks for looking into it. >> >> I am copy-pasting at the end of this email a minimal reproduction. It >> calculates a checksum and prints it. The correct value is f8d9. If you compile >> it with -O0 or -O3 -fno-strict-aliasing, you will get the correct value. If >> you compile it with gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0 and -O3, you will >> get f8e8. You can also try it on https://godbolt.org/ and see how different >> versions behave. >> >> My understanding is that the code violates the C standard >> (https://stackoverflow.com/a/99010). >> > > Thanks for the sample code below, I copied to the godbolt: > https://godbolt.org/z/6fMK19 > > In gcc 10, the checksum calculation is done during compilation (when > optimization is enabled) and the value is returned directly: > mov    $0xffed,%esi > > Since a calculation is happening I assume the compiler knows about the aliasing > and OK with it. > > But that optimized calculation seems wrong, when it is disabled [1] the checksum > is correct again. > > [1] all following seems helping to disable compile time calculation > - disabling optimization > - putting a compiler barrier > - putting a 'printf' inside 'ip_sum()' > - fno-strict-aliasing > > gcc 8 & 9 is not doing this compile time calculation, hence they are not affected. > > This feels like an optimization issue in gcc10, but not sure exactly on the root > cause, and how to disable it properly in our case. > As checked with the Harry, latest finding is gcc 10 left out any _non_ uint16_t type variable in sturct during its compile time calculation. Not sure if it is because of broken aliasing or gcc defect, I will report the issue. Meanwhile for short time solution, can you please try force uninline the 'ip_sum()' and try? >> --- cut here --- >> >> #include >> #include >> #include >> #include >> >> struct rte_ipv4_hdr { >>     uint8_t  version_ihl; >>     uint8_t  type_of_service; >>     uint16_t total_length; >>     uint16_t packet_id; >>     uint16_t fragment_offset; >>     uint8_t  time_to_live; >>     uint8_t  next_proto_id; >>     uint16_t hdr_checksum; >>     uint32_t src_addr; >>     uint32_t dst_addr; >> }; >> >> static inline uint16_t ip_sum(const uint16_t *hdr, int hdr_len) >> { >>     uint32_t sum = 0; >> >>     while (hdr_len > 1) >>     { >>         sum += *hdr++; >>         if (sum & 0x80000000) >>             sum = (sum & 0xFFFF) + (sum >> 16); >>         hdr_len -= 2; >>     } >> >>     while (sum >> 16) >>         sum = (sum & 0xFFFF) + (sum >> 16); >> >>     return ~sum; >> } >> >> static void pkt_burst_flow_gen(void) >> { >>     struct rte_ipv4_hdr *ip_hdr = (struct rte_ipv4_hdr *) malloc(4096); >>     memset(ip_hdr, 0, sizeof(*ip_hdr)); >>     ip_hdr->version_ihl    = 1; >>     ip_hdr->type_of_service    = 2; >>     ip_hdr->fragment_offset    = 3; >>     ip_hdr->time_to_live    = 4; >>     ip_hdr->next_proto_id    = 5; >>     ip_hdr->packet_id    = 6; >>     ip_hdr->src_addr    = 7; >>     ip_hdr->dst_addr    = 8; >>     ip_hdr->total_length    = 9; >>     ip_hdr->hdr_checksum    = ip_sum((uint16_t *)ip_hdr, sizeof(*ip_hdr)); >>     printf("%x\n", ip_hdr->hdr_checksum); >> } >> >> int main(void) >> { >>     pkt_burst_flow_gen(); >>     return 0; >> } >> >