From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Subject: Re: TCP performance problems - GSO/TSO, MSS, 8139cp related Date: Fri, 11 Nov 2016 22:44:27 +0000 Message-ID: <20161111224427.GG1041@n2100.armlinux.org.uk> References: <20161111210500.GE1041@n2100.armlinux.org.uk> <1478899423.3892.7.camel@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: netdev@vger.kernel.org, qemu-devel@nongnu.org To: David Woodhouse Return-path: Received: from pandora.armlinux.org.uk ([78.32.30.218]:41176 "EHLO pandora.armlinux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933600AbcKKWoe (ORCPT ); Fri, 11 Nov 2016 17:44:34 -0500 Content-Disposition: inline In-Reply-To: <1478899423.3892.7.camel@infradead.org> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, Nov 11, 2016 at 09:23:43PM +0000, David Woodhouse wrote: > It's also *fairly* unlikely that the kernel in the guest has developed > a bug and isn't setting gso_size sanely. I'm more inclined to suspect > that qemu isn't properly emulating those bits. But at first glance at > the code, it looks like *that's* been there for the last decade too... I take issue with that, having looked at the qemu rtl8139 code: if ((txdw0 & CP_TX_LGSEN) && ip_protocol == IP_PROTO_TCP) { int large_send_mss = (txdw0 >> 16) & CP_TC_LGSEN_MSS_MASK; DPRINTF("+++ C+ mode offloaded task TSO MTU=%d IP data %d " "frame data %d specified MSS=%d\n", ETH_MTU, ip_data_len, saved_size - ETH_HLEN, large_send_mss); That's the only reference to "large_send_mss" there, other than that, the MSS value that gets stuck into the field by 8139cp.c is completely unused. Instead, qemu does this: eth_payload_data = saved_buffer + ETH_HLEN; eth_payload_len = saved_size - ETH_HLEN; ip = (ip_header*)eth_payload_data; hlen = IP_HEADER_LENGTH(ip); ip_data_len = be16_to_cpu(ip->ip_len) - hlen; tcp_header *p_tcp_hdr = (tcp_header*)(eth_payload_data + hlen); int tcp_hlen = TCP_HEADER_DATA_OFFSET(p_tcp_hdr); /* ETH_MTU = ip header len + tcp header len + payload */ int tcp_data_len = ip_data_len - tcp_hlen; int tcp_chunk_size = ETH_MTU - hlen - tcp_hlen; for (tcp_send_offset = 0; tcp_send_offset < tcp_data_len; tcp_send_offset += tcp_chunk_size) { It uses a fixed value of ETH_MTU to calculate the size of the TCP data chunks, and this is not surprisingly the well known: #define ETH_MTU 1500 Qemu seems to be buggy - it ignores the MSS value, and always tries to send 1500 byte frames. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.