* Russell King - ARM Linux (li...@armlinux.org.uk) wrote:
> 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.
cc'ing in Stefan who last touched that code and Jason and Vlad who
know the net code.
Dave
> --
> 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.
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
