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 5620DA0524; Thu, 7 Jan 2021 15:20:37 +0100 (CET) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id D1F9E140F90; Thu, 7 Jan 2021 15:20:36 +0100 (CET) Received: from smtp-fw-2101.amazon.com (smtp-fw-2101.amazon.com [72.21.196.25]) by mails.dpdk.org (Postfix) with ESMTP id 5727F140F8B for ; Thu, 7 Jan 2021 15:20:35 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazon201209; t=1610029236; x=1641565236; h=to:cc:references:from:message-id:date:mime-version: in-reply-to:content-transfer-encoding:subject; bh=Jgx/EysfrVIdVlvueJ5Vg7eHsPSDQ5BYnQGMFKw6uRw=; b=U4S1lzujFFzbRmUllqvA7AYw61NXIRqyQzg8Z2Y2y6oCsKYKRN41jpCx zYiiaUwdv3WHMuLjGYf3PklDMOlmBYspq+OeV9sxAwmjnEenLmFNfQu1P rRiOgWaWTpyrZ66dSeHR3Qq5jQwsBbInhrTjY/xZqc12z+26EfF0y4F3J w=; X-IronPort-AV: E=Sophos;i="5.79,329,1602547200"; d="scan'208";a="73393583" Received: from iad12-co-svc-p1-lb1-vlan3.amazon.com (HELO email-inbound-relay-2b-a7fdc47a.us-west-2.amazon.com) ([10.43.8.6]) by smtp-border-fw-out-2101.iad2.amazon.com with ESMTP; 07 Jan 2021 14:20:34 +0000 Received: from EX13MTAUWC002.ant.amazon.com (pdx1-ws-svc-p6-lb9-vlan2.pdx.amazon.com [10.236.137.194]) by email-inbound-relay-2b-a7fdc47a.us-west-2.amazon.com (Postfix) with ESMTPS id EF382C0937; Thu, 7 Jan 2021 14:20:32 +0000 (UTC) Received: from EX13D12UWC002.ant.amazon.com (10.43.162.253) by EX13MTAUWC002.ant.amazon.com (10.43.162.240) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Thu, 7 Jan 2021 14:20:32 +0000 Received: from [192.168.4.85] (10.43.161.224) by EX13D12UWC002.ant.amazon.com (10.43.162.253) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Thu, 7 Jan 2021 14:20:31 +0000 To: Ferruh Yigit , Wenzhuo Lu , Beilei Xing , Bernard Iremonger CC: , Stephen Hemminger , "George Prekas (prekageo)" 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> From: George Prekas Message-ID: Date: Thu, 7 Jan 2021 08:20:29 -0600 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <6a84a356-191a-fca6-607f-3caf88eb3da6@intel.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.43.161.224] X-ClientProxiedBy: EX13D44UWC002.ant.amazon.com (10.43.162.169) To EX13D12UWC002.ant.amazon.com (10.43.162.253) 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 5:32 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. According to https://gcc.gnu.org/bugs/: "if compiling with -fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations makes a difference ... then your code is probably not correct" > > 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. I just checked gcc 8.3 and gcc 9.3 on godbolt and I got f8e8 (which is wrong; the correct is f8d9). > > 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. I've tried with __attribute__ ((noinline)) and it fixes the problem. But keep in mind that we are dealing with broken C code. This attribute just prevents the optimization that reveals the problem. It does not guarantee that the problem will not reappear in a future compiler version. I've also tried to use a union as suggested by Stephen Hemminger and it works correctly but it requires significant code changes: you have to copy paste the IP header structure inside a union and access it only through the union. As a side note, here is a piece of opinion from Linus Torvalds regarding strict aliasing: https://lkml.org/lkml/2018/6/5/769 DPDK already uses -fno-strict-aliasing for librte_node and librte_vhost. > >> --- 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; >> } >> >