Re: TCP performance problems - GSO/TSO, MSS, 8139cp related

[Russell King - ARM Linux <linux@armlinux.org.uk>]

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.

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