Date: Thu, 5 Nov 2015 11:00:57 -0500 From: Larry Baird <lab@gta.com> To: Roger Pau Monn?? <roger.pau@citrix.com> Cc: freebsd-xen@freebsd.org, alans@spectralogic.com, johns@spectralogic.com, ken@FreeBSD.org Subject: Re: Checksum forwarding issue on XEN Message-ID: <20151105160057.GA2268@gta.com> In-Reply-To: <563B72B2.6060308@citrix.com> References: <20151103201250.GA92469@gta.com> <563B72B2.6060308@citrix.com>
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Roger, > Adding the persons that contributed that code in case they can shed some > light. > > El 03/11/15 a les 21.12, Larry Baird ha escrit: > > Has anybody made any progress on "Bug 188261 - [xen] FreeBSD DomU PVHVM > > guests cannot 'route' traffic for other Xen PV guests on same Dom0 Host." > > (https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=188261)? > > > > The code for checksum calculation in the function xnb_add_mbuf_cksum() looks > > suspect. > > > > switch (iph->ip_p) { > > case IPPROTO_TCP: > > if (mbufc->m_pkthdr.csum_flags & CSUM_IP_VALID) { > > size_t tcplen = ntohs(iph->ip_len) - sizeof(struct ip); > > struct tcphdr *th = (struct tcphdr*)(iph + 1); > > th->th_sum = in_pseudo(iph->ip_src.s_addr, > > iph->ip_dst.s_addr, htons(IPPROTO_TCP + tcplen)); > > th->th_sum = in_cksum_skip(mbufc, > > sizeof(struct ether_header) + ntohs(iph->ip_len), > > sizeof(struct ether_header) + (iph->ip_hl << 2)); > > } > > break; > > case IPPROTO_UDP: > > if (mbufc->m_pkthdr.csum_flags & CSUM_IP_VALID) { > > size_t udplen = ntohs(iph->ip_len) - sizeof(struct ip); > > struct udphdr *uh = (struct udphdr*)(iph + 1); > > uh->uh_sum = in_pseudo(iph->ip_src.s_addr, > > iph->ip_dst.s_addr, htons(IPPROTO_UDP + udplen)); > > uh->uh_sum = in_cksum_skip(mbufc, > > sizeof(struct ether_header) + ntohs(iph->ip_len), > > sizeof(struct ether_header) + (iph->ip_hl << 2)); > > } > > break; > > default: > > break; > > } > > > > > > Both in_pseudo() and in_cksum_skip() set the same checksum. Does this > > make since to anybody? > > The bug you are referring to affects FreeBSD when running as a guest > using xen-netfront, but the code snipped above and the function > referenced (xnb_add_mbuf_cksum) is only used on FreeBSD when running as > a host (AKA Dom0) by xen-netback. > > TBH, I don't know that much about FreeBSD network subsystem to have an > opinion, but it certainly looks weird. Patches are welcome :). Xyper-V has a similar forward issue. I found they were misusing csum_flags and were always attempting to do checksum offloading if CSUM_IP_VALID was set. I have given them a patch that fixes the issue. I was hoping that Xen's issue was similar. I found the issue above by looking at all uses of csum_flags in sys/dev/xen. It is hard to tell what the correct fix is, without fulling understand the protocal used when communicating between backend and frontend of Xen. I am sure issue with XEN guest forwarding has to with checksum offloading. If I am not misinterpreting your comments, I can ignore code in netback and concentrate on code in netfront when trying to understand what is going wrong. Thank you for some insite, Larry -- ------------------------------------------------------------------------ Larry Baird Global Technology Associates, Inc. 1992-2012 | http://www.gta.com Celebrating Twenty Years of Software Innovation | Orlando, FL Email: lab@gta.com | TEL 407-380-0220
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