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Re: [Qemu-devel] [ RFC Patch v6 1/3] virtio-net rsc: support coalescing


From: Wei Xu
Subject: Re: [Qemu-devel] [ RFC Patch v6 1/3] virtio-net rsc: support coalescing ipv4 tcp traffic
Date: Fri, 3 Jun 2016 00:16:13 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

        On 2016年05月30日 12:20, Jason Wang wrote:


On 2016年05月29日 00:37, address@hidden wrote:
From: Wei Xu <address@hidden>

All the data packets in a tcp connection will be cached to a big buffer
in every receive interval, and will be sent out via a timer, the
'virtio_net_rsc_timeout' controls the interval, the value will
influent the
performance and response of tcp connection extremely, 50000(50us) is a
experience value to gain a performance improvement, since the whql test
sends packets every 100us, so '300000(300us)' can pass the test case,
this is also the default value, it's tunable via the command line
parameter 'rsc_interval' with 'virtio-net-pci' device, for example, below
parameter is to launch a guest with interval set as '500000'.

Does the value make sense if it was smaller than 1us? If not, why not
just make the unit to be 1us?
It's an experience value, 500us is a good candidate for netperf throughput test, too short interval less than 50 us will gain an obvious penalty AFAIK, this is caused only few packets can be coalesced and the cycles wasted for rsc itself.

'virtio-net-pci,netdev=hostnet1,bus=pci.0,id=net1,mac=00,rsc_interval=500000'

will

The timer will only be triggered if the packets pool is not empty,
and it'll drain off all the cached packets.

'NetRscChain' is used to save the segments of different protocols in a
VirtIONet device.

The main handler of TCP includes TCP window update, duplicated ACK check
and the real data coalescing if the new segment passed sanity check
and is identified as an 'wanted' one.

An 'wanted' segment means:
1. Segment is within current window and the sequence is the expected one.
2. 'ACK' of the segment is in the valid window.

Sanity check includes:
1. Incorrect version in IP header
2. IP options & IP fragment
3. Not a TCP packets
4. Sanity size check to prevent buffer overflow attack.

There maybe more cases should be considered such as ip identification
other
flags, while it broke the test because windows set it to the same even
it's
not a fragment.

Normally it includes 2 typical ways to handle a TCP control flag,
'bypass'
and 'finalize', 'bypass' means should be sent out directly, and
'finalize'
means the packets should also be bypassed, and this should be done
after searching for the same connection packets in the pool and sending
all of them out, this is to avoid out of data.

All the 'SYN' packets will be bypassed since this always begin a new'
connection, other flags such 'FIN/RST' will trigger a finalization,
because
this normally happens upon a connection is going to be closed, an
'URG' packet
also finalize current coalescing unit.

Statistics can be used to monitor the basic coalescing status, the
'out of order'
and 'out of window' means how many retransmitting packets, thus
describe the
performance intuitively.

We usually don't add device specific monitor command. Maybe a new
control vq command ethtool -S in guest in the future. I was thinking of
removing those counters since it was never used in this series.
Yes, that's a good idea, actually i'm doubting whether this feature should be a guest feature or a host feature, the spec says it should be more like a guest feature, but it's provided as a host built-in feature, so how and where to examine it is a problem. I'm using gdb to debugging it currently, normally i would check this counter directly via debug command, and the statistics is quite useful for troubleshooting, it'll be optional when this feature is geting more and more robust, can we keep it and leave if should keep it or where to display it along with qa's testing?


Signed-off-by: Wei Xu <address@hidden>
---
  hw/net/virtio-net.c                         | 498
+++++++++++++++++++++++++++-
  include/hw/virtio/virtio-net.h              |   2 +
  include/hw/virtio/virtio.h                  |  75 +++++
  include/standard-headers/linux/virtio_net.h |   1 +

For RFC, it's ok. But for formal patch, this is not the correct way to
modify Linux headers. There's a script in
scripts/update-linux-headers.sh which was used to sync it from Linux
source. This means, it must be merged in Linux or at least in
maintainer's tree first.

  4 files changed, 575 insertions(+), 1 deletion(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 5798f87..b3bb63b 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -15,10 +15,12 @@
  #include "qemu/iov.h"
  #include "hw/virtio/virtio.h"
  #include "net/net.h"
+#include "net/eth.h"
  #include "net/checksum.h"
  #include "net/tap.h"
  #include "qemu/error-report.h"
  #include "qemu/timer.h"
+#include "qemu/sockets.h"
  #include "hw/virtio/virtio-net.h"
  #include "net/vhost_net.h"
  #include "hw/virtio/virtio-bus.h"
@@ -38,6 +40,25 @@
  #define endof(container, field) \
      (offsetof(container, field) + sizeof(((container *)0)->field))
+#define VIRTIO_NET_IP4_ADDR_SIZE   8        /* ipv4 saddr + daddr */
+#define VIRTIO_NET_TCP_PORT_SIZE   4        /* sport + dport */
+
+#define VIRTIO_NET_TCP_FLAG         0x3F
+#define VIRTIO_NET_TCP_HDR_LENGTH   0xF000
+
+/* IPv4 max payload, 16 bits in the header */
+#define VIRTIO_NET_MAX_IP4_PAYLOAD (65535 - sizeof(struct ip_header))
+#define VIRTIO_NET_MAX_TCP_PAYLOAD 65535

Is this still true if window scaling is enabled? And need to make sure
your ack/seq processing can work for window scaling.

+
+/* header length value in ip header without option */
+#define VIRTIO_NET_IP4_HEADER_LENGTH 5
+
+/* Purge coalesced packets timer interval, This value affects the
performance
+   a lot, and should be tuned carefully, '300000'(300us) is the
recommended
+   value to pass the WHQL test, '50000' can gain 2x netperf
throughput with
+   tso/gso/gro 'off'. */
+#define VIRTIO_NET_RSC_INTERVAL  300000

Do we need a new property for this value?
Yes, i added this already.
+    DEFINE_PROP_UINT32("rsc_interval", VirtIONet, rsc_timeout,
+                      VIRTIO_NET_RSC_INTERVAL),

+
  typedef struct VirtIOFeature {
      uint32_t flags;
      size_t end;
@@ -1089,7 +1110,8 @@ static int receive_filter(VirtIONet *n, const
uint8_t *buf, int size)
      return 0;
  }
-static ssize_t virtio_net_receive(NetClientState *nc, const uint8_t
*buf, size_t size)
+static ssize_t virtio_net_do_receive(NetClientState *nc,
+                                     const uint8_t *buf, size_t size)
  {
      VirtIONet *n = qemu_get_nic_opaque(nc);
      VirtIONetQueue *q = virtio_net_get_subqueue(nc);
@@ -1685,6 +1707,474 @@ static int virtio_net_load_device(VirtIODevice
*vdev, QEMUFile *f,
      return 0;
  }
+static void virtio_net_rsc_extract_unit4(NetRscChain *chain,
+                                         const uint8_t *buf,
NetRscUnit* unit)
+{
+    uint16_t hdr_len;
+    uint16_t ip_hdrlen;
+    struct ip_header *ip;
+
+    hdr_len = chain->n->guest_hdr_len;
+    ip = (struct ip_header *)(buf + hdr_len + sizeof(struct
eth_header));
+    unit->ip = (void *)ip;
+    ip_hdrlen = (ip->ip_ver_len & 0xF) << 2;
+    unit->ip_plen = &ip->ip_len;
+    unit->tcp = (struct tcp_header *)(((uint8_t *)unit->ip) +
ip_hdrlen);
+    unit->tcp_hdrlen = (htons(unit->tcp->th_offset_flags) & 0xF000)
>> 10;
+    unit->payload = htons(*unit->ip_plen) - ip_hdrlen -
unit->tcp_hdrlen;
+}
+
+static void virtio_net_rsc_ipv4_checksum(struct virtio_net_hdr *vhdr,
+                                         struct ip_header *ip)
+{
+    uint32_t sum;
+
+    ip->ip_sum = 0;
+    sum = net_checksum_add_cont(sizeof(struct ip_header), (uint8_t
*)ip, 0);
+    ip->ip_sum = cpu_to_be16(net_checksum_finish(sum));
+    vhdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
+    vhdr->flags |= VIRTIO_NET_HDR_F_DATA_VALID;
+}
+
+static size_t virtio_net_rsc_drain_seg(NetRscChain *chain, NetRscSeg
*seg)
+{
+    int ret;
+    struct virtio_net_hdr *h;
+
+    h = (struct virtio_net_hdr *)seg->buf;
+    virtio_net_rsc_ipv4_checksum(h, seg->unit.ip);
+    ret = virtio_net_do_receive(seg->nc, seg->buf, seg->size);
+    QTAILQ_REMOVE(&chain->buffers, seg, next);
+    g_free(seg->buf);
+    g_free(seg);
+
+    return ret;
+}
+
+static void virtio_net_rsc_purge(void *opq)
+{
+    NetRscSeg *seg, *rn;
+    NetRscChain *chain = (NetRscChain *)opq;
+
+    QTAILQ_FOREACH_SAFE(seg, &chain->buffers, next, rn) {
+        if (virtio_net_rsc_drain_seg(chain, seg) == 0) {
+            chain->stat.purge_failed++;
+            continue;

Why need this?
ah., We have discussed this before, there maybe momentary congestion and can be resumed quickly for the guest, so it's try to continue receiving rather then drop the cached packets.

+        }
+    }
+
+    chain->stat.timer++;
+    if (!QTAILQ_EMPTY(&chain->buffers)) {
+        timer_mod(chain->drain_timer,
+              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) +
chain->n->rsc_timeout);

Consider the timer is invisible to guest, why use QEMU_CLOCK_VIRTUAL?
Yes, it's invisible to the guest, should so should i choose a host or timer here?

+    }
+}
+
+static void virtio_net_rsc_cleanup(VirtIONet *n)
+{
+    NetRscChain *chain, *rn_chain;
+    NetRscSeg *seg, *rn_seg;
+
+    QTAILQ_FOREACH_SAFE(chain, &n->rsc_chains, next, rn_chain) {
+        QTAILQ_FOREACH_SAFE(seg, &chain->buffers, next, rn_seg) {
+            QTAILQ_REMOVE(&chain->buffers, seg, next);
+            g_free(seg->buf);
+            g_free(seg);
+        }
+
+        timer_del(chain->drain_timer);
+        timer_free(chain->drain_timer);
+        QTAILQ_REMOVE(&n->rsc_chains, chain, next);
+        g_free(chain);
+    }
+}
+
+static void virtio_net_rsc_cache_buf(NetRscChain *chain,
NetClientState *nc,
+                                     const uint8_t *buf, size_t size)
+{
+    uint16_t hdr_len;
+    NetRscSeg *seg;
+
+    hdr_len = chain->n->guest_hdr_len;
+    seg = g_malloc(sizeof(NetRscSeg));
+    seg->buf = g_malloc(hdr_len + sizeof(struct eth_header)\
+                   + VIRTIO_NET_MAX_TCP_PAYLOAD);
+    memcpy(seg->buf, buf, size);
+    seg->size = size;
+    seg->packets = 1;
+    seg->dup_ack = 0;
+    seg->is_coalesced = 0;
+    seg->nc = nc;
+
+    QTAILQ_INSERT_TAIL(&chain->buffers, seg, next);
+    chain->stat.cache++;
+
+    virtio_net_rsc_extract_unit4(chain, seg->buf, &seg->unit);
+}
+
+static int32_t virtio_net_rsc_handle_ack(NetRscChain *chain,
+                                         NetRscSeg *seg, const
uint8_t *buf,
+                                         struct tcp_header *n_tcp,
+                                         struct tcp_header *o_tcp)
+{
+    uint32_t nack, oack;
+    uint16_t nwin, owin;
+
+    nack = htonl(n_tcp->th_ack);
+    nwin = htons(n_tcp->th_win);
+    oack = htonl(o_tcp->th_ack);
+    owin = htons(o_tcp->th_win);
+
+    if ((nack - oack) >= VIRTIO_NET_MAX_TCP_PAYLOAD) {
+        chain->stat.ack_out_of_win++;
+        return RSC_FINAL;
+    } else if (nack == oack) {
+        /* duplicated ack or window probe */
+        if (nwin == owin) {
+            /* duplicated ack, add dup ack count due to whql test up
to 1 */
+            chain->stat.dup_ack++;
+            return RSC_FINAL;
+        } else {
+            /* Coalesce window update */
+            o_tcp->th_win = n_tcp->th_win;
+            chain->stat.win_update++;
+            return RSC_COALESCE;
+        }
+    } else {
+        /* pure ack, go to 'C', finalize*/
+        chain->stat.pure_ack++;
+        return RSC_FINAL;
+    }
+}
+
+static int32_t virtio_net_rsc_coalesce_data(NetRscChain *chain,
+                                            NetRscSeg *seg, const
uint8_t *buf,
+                                            NetRscUnit *n_unit)
+{
+    void *data;
+    uint16_t o_ip_len;
+    uint32_t nseq, oseq;
+    NetRscUnit *o_unit;
+
+    o_unit = &seg->unit;
+    o_ip_len = htons(*o_unit->ip_plen);
+    nseq = htonl(n_unit->tcp->th_seq);
+    oseq = htonl(o_unit->tcp->th_seq);
+
+    /* out of order or retransmitted. */
+    if ((nseq - oseq) > VIRTIO_NET_MAX_TCP_PAYLOAD) {
+        chain->stat.data_out_of_win++;
+        return RSC_FINAL;
+    }
+
+    data = ((uint8_t *)n_unit->tcp) + n_unit->tcp_hdrlen;
+    if (nseq == oseq) {
+        if ((o_unit->payload == 0) && n_unit->payload) {
+            /* From no payload to payload, normal case, not a dup ack
or etc */
+            chain->stat.data_after_pure_ack++;
+            goto coalesce;
+        } else {
+            return virtio_net_rsc_handle_ack(chain, seg, buf,
+                                             n_unit->tcp, o_unit->tcp);
+        }
+    } else if ((nseq - oseq) != o_unit->payload) {
+        /* Not a consistent packet, out of order */
+        chain->stat.data_out_of_order++;
+        return RSC_FINAL;
+    } else {
+coalesce:
+        if ((o_ip_len + n_unit->payload) > chain->max_payload) {
+            chain->stat.over_size++;
+            return RSC_FINAL;
+        }
+
+        /* Here comes the right data, the payload length in v4/v6 is
different,
+           so use the field value to update and record the new data
len */
+        o_unit->payload += n_unit->payload; /* update new data len */
+
+        /* update field in ip header */
+        *o_unit->ip_plen = htons(o_ip_len + n_unit->payload);
+
+        /* Bring 'PUSH' big, the whql test guide says 'PUSH' can be
coalesced
+           for windows guest, while this may change the behavior for
linux
+           guest. */
+        o_unit->tcp->th_offset_flags = n_unit->tcp->th_offset_flags;
+
+        o_unit->tcp->th_ack = n_unit->tcp->th_ack;
+        o_unit->tcp->th_win = n_unit->tcp->th_win;
+
+        memmove(seg->buf + seg->size, data, n_unit->payload);
+        seg->size += n_unit->payload;
+        seg->packets++;
+        chain->stat.coalesced++;
+        return RSC_COALESCE;
+    }
+}
+
+static int32_t virtio_net_rsc_coalesce4(NetRscChain *chain, NetRscSeg
*seg,
+                                        const uint8_t *buf, size_t size,
+                                        NetRscUnit *unit)
+{
+    struct ip_header *ip1, *ip2;
+
+    ip1 = (struct ip_header *)(unit->ip);
+    ip2 = (struct ip_header *)(seg->unit.ip);
+    if ((ip1->ip_src ^ ip2->ip_src) || (ip1->ip_dst ^ ip2->ip_dst)
+        || (unit->tcp->th_sport ^ seg->unit.tcp->th_sport)
+        || (unit->tcp->th_dport ^ seg->unit.tcp->th_dport)) {
+        chain->stat.no_match++;
+        return RSC_NO_MATCH;
+    }
+
+    return virtio_net_rsc_coalesce_data(chain, seg, buf, unit);
+}
+
+/* Pakcets with 'SYN' should bypass, other flag should be sent after
drain
+ * to prevent out of order */
+static int virtio_net_rsc_tcp_ctrl_check(NetRscChain *chain,
+                                         struct tcp_header *tcp)
+{
+    uint16_t tcp_hdr;
+    uint16_t tcp_flag;
+
+    tcp_flag = htons(tcp->th_offset_flags);
+    tcp_hdr = (tcp_flag & VIRTIO_NET_TCP_HDR_LENGTH) >> 10;
+    tcp_flag &= VIRTIO_NET_TCP_FLAG;
+    tcp_flag = htons(tcp->th_offset_flags) & 0x3F;
+    if (tcp_flag & TH_SYN) {
+        chain->stat.tcp_syn++;
+        return RSC_BYPASS;
+    }
+
+    if (tcp_flag & (TH_FIN | TH_URG | TH_RST)) {
+        chain->stat.tcp_ctrl_drain++;
+        return RSC_FINAL;
+    }
+
+    if (tcp_hdr > sizeof(struct tcp_header)) {
+        chain->stat.tcp_all_opt++;
+        return RSC_FINAL;
+    }
+
+    return RSC_WANT;
+}
+
+static bool virtio_net_rsc_empty_cache(NetRscChain *chain,
NetClientState *nc,
+                                       const uint8_t *buf, size_t size)
+{

The name is really confusing, it in fact cache the buf if the cache is
empty. So I guess what you mean is "virtio_net_rsc_cache_if_empty()?".
But the question is why need a specific function like this, can we
simply add the logic to virtio_net_rsc_do_coalesce()? (And looks like
most of the logic has been there, except for timer and counter.
OK.

+    if (QTAILQ_EMPTY(&chain->buffers)) {
+        chain->stat.empty_cache++;
+        virtio_net_rsc_cache_buf(chain, nc, buf, size);
+        timer_mod(chain->drain_timer,
+              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) +
chain->n->rsc_timeout);
+        return 1;
+    }
+
+    return 0;
+}
+
+static size_t virtio_net_rsc_do_coalesce(NetRscChain *chain,
NetClientState *nc,
+                                         const uint8_t *buf, size_t
size,
+                                         NetRscUnit *unit)
+{
+    int ret;
+    NetRscSeg *seg, *nseg;
+
+    QTAILQ_FOREACH_SAFE(seg, &chain->buffers, next, nseg) {
+        ret = virtio_net_rsc_coalesce4(chain, seg, buf, size, unit);
+
+        if (ret == RSC_FINAL) {
+            if (virtio_net_rsc_drain_seg(chain, seg) == 0) {
+                /* Send failed */
+                chain->stat.final_failed++;
+                return 0;
+            }
+
+            /* Send current packet */
+            return virtio_net_do_receive(nc, buf, size);
+        } else if (ret == RSC_NO_MATCH) {
+            continue;
+        } else {
+            /* Coalesced, mark coalesced flag to tell calc cksum for
ipv4 */
+            seg->is_coalesced = 1;
+            return size;
+        }
+    }
+
+    chain->stat.no_match_cache++;
+    virtio_net_rsc_cache_buf(chain, nc, buf, size);
+    return size;
+}
+
+/* Drain a connection data, this is to avoid out of order segments */
+static size_t virtio_net_rsc_drain_flow(NetRscChain *chain,
NetClientState *nc,
+                                        const uint8_t *buf, size_t size,
+                                        uint16_t ip_start, uint16_t
ip_size,
+                                        uint16_t tcp_port, uint16_t
port_size)
+{
+    NetRscSeg *seg, *nseg;
+
+    QTAILQ_FOREACH_SAFE(seg, &chain->buffers, next, nseg) {
+        if (memcmp(buf + ip_start, seg->buf + ip_start, ip_size)
+            || memcmp(buf + tcp_port, seg->buf + tcp_port, port_size)) {

The header comparison is rather similar to what you've done in
virtio_net_rsc_coalesce4/6(). Any chance to reduce the duplication?
OK.

+            continue;
+        }
+        if (virtio_net_rsc_drain_seg(chain, seg) == 0) {
+            chain->stat.drain_failed++;
+        }
+
+        break;
+    }
+
+    return virtio_net_do_receive(nc, buf, size);
+}
+
+static int32_t virtio_net_rsc_sanity_check4(NetRscChain *chain,
+                                            struct ip_header *ip,
+                                            const uint8_t *buf,
size_t size)
+{
+    uint16_t ip_len;
+    uint16_t hdr_len;
+
+    hdr_len = chain->n->guest_hdr_len;
+    if (size < (hdr_len + sizeof(struct eth_header) + sizeof(struct
ip_header)
+        + sizeof(struct tcp_header))) {

It's interesting that you don't have a stat counter for this case.
Thanks.

+        return RSC_BYPASS;
+    }
+
+    /* Not an ipv4 one */
+    if (((ip->ip_ver_len & 0xF0) >> 4) != IP_HEADER_VERSION_4) {
+        chain->stat.ip_option++;
+        return RSC_BYPASS;
+    }
+
+    /* Don't handle packets with ip option */
+    if (VIRTIO_NET_IP4_HEADER_LENGTH != (ip->ip_ver_len & 0xF)) {

(ip-> ...) != VIRTIO_NET_IP4_HEADER_LENGTH
OK.

+        chain->stat.ip_option++;
+        return RSC_BYPASS;
+    }
+
+    if (ip->ip_p != IPPROTO_TCP) {
+        chain->stat.bypass_not_tcp++;
+        return RSC_BYPASS;
+    }
+
+    /* Don't handle packets with ip fragment */
+    if (!(htons(ip->ip_off) & IP_DF)) {
+        chain->stat.ip_frag++;
+        return RSC_BYPASS;
+    }
+
+    ip_len = htons(ip->ip_len);
+    if (ip_len < (sizeof(struct ip_header) + sizeof(struct tcp_header))
+        || ip_len > (size - hdr_len - sizeof(struct eth_header))) {
+        chain->stat.ip_hacked++;
+        return RSC_BYPASS;
+    }
+
+    return RSC_WANT;
+}
+
+static size_t virtio_net_rsc_receive4(NetRscChain *chain,
NetClientState* nc,
+                                      const uint8_t *buf, size_t size)
+{
+    int32_t ret;
+    uint16_t hdr_len;
+    NetRscUnit unit;
+
+    hdr_len = ((VirtIONet *)(chain->n))->guest_hdr_len;
+    virtio_net_rsc_extract_unit4(chain, buf, &unit);

Is this safe for you do header analysis before doing sanity check?
looks, there should be unsafe case here, will double check it.

+    if (RSC_WANT != virtio_net_rsc_sanity_check4(chain, unit.ip, buf,
size)) {

virtio_net_rsc_sanity_check4() = RSC_WANT please. I've pointed out this
kind of style issues several times.

+        return virtio_net_do_receive(nc, buf, size);
+    }
+
+    ret = virtio_net_rsc_tcp_ctrl_check(chain, unit.tcp);
+    if (ret == RSC_BYPASS) {
+        return virtio_net_do_receive(nc, buf, size);
+    } else if (ret == RSC_FINAL) {
+        return virtio_net_rsc_drain_flow(chain, nc, buf, size,
+                ((hdr_len + sizeof(struct eth_header)) + 12),
+                VIRTIO_NET_IP4_ADDR_SIZE,
+                hdr_len + sizeof(struct eth_header) + sizeof(struct
ip_header),
+                VIRTIO_NET_TCP_PORT_SIZE);
+    }
+
+    if (virtio_net_rsc_empty_cache(chain, nc, buf, size)) {
+        return size;
+    }
+
+    return virtio_net_rsc_do_coalesce(chain, nc, buf, size, &unit);
+}
+
+static NetRscChain *virtio_net_rsc_lookup_chain(VirtIONet * n,
+                                                NetClientState *nc,
+                                                uint16_t proto)
+{
+    NetRscChain *chain;
+
+    if (proto != (uint16_t)ETH_P_IP) {
+        return NULL;
+    }
+
+    QTAILQ_FOREACH(chain, &n->rsc_chains, next) {
+        if (chain->proto == proto) {
+            return chain;
+        }
+    }
+
+    chain = g_malloc(sizeof(*chain));
+    chain->n = n;
+    chain->proto = proto;
+    chain->max_payload = VIRTIO_NET_MAX_IP4_PAYLOAD;
+    chain->gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
+    chain->drain_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
+                                      virtio_net_rsc_purge, chain);
+    memset(&chain->stat, 0, sizeof(chain->stat));
+
+    QTAILQ_INIT(&chain->buffers);
+    QTAILQ_INSERT_TAIL(&n->rsc_chains, chain, next);
+
+    return chain;
+}
+
+static ssize_t virtio_net_rsc_receive(NetClientState *nc,
+                                      const uint8_t *buf, size_t size)
+{
+    uint16_t proto;
+    NetRscChain *chain;
+    struct eth_header *eth;
+    VirtIONet *n;
+
+    n = qemu_get_nic_opaque(nc);
+    if (size < (n->guest_hdr_len + sizeof(struct eth_header))) {

Here should use host_hdr_len, no?
Not understanding the code well, could you please give a typical usage of in what kind of case will the host_hdr_len be different with guest_hdr_len? i was thinking they are equal in most cases, is it caused by different feature capability such as mergeable buffers, etc?

+        return virtio_net_do_receive(nc, buf, size);
+    }
+
+    eth = (struct eth_header *)(buf + n->guest_hdr_len);
+    proto = htons(eth->h_proto);
+
+    chain = virtio_net_rsc_lookup_chain(n, nc, proto);
+    if (!chain) {
+        return virtio_net_do_receive(nc, buf, size);
+    } else {
+        chain->stat.received++;
+        return virtio_net_rsc_receive4(chain, nc, buf, size);
+    }
+}
+
+static ssize_t virtio_net_receive(NetClientState *nc,
+                                  const uint8_t *buf, size_t size)
+{
+    VirtIONet *n;
+
+    n = qemu_get_nic_opaque(nc);
+    if (n->host_features & (1ULL << VIRTIO_NET_F_GUEST_RSC)) {
+        return virtio_net_rsc_receive(nc, buf, size);
+    } else {
+        return virtio_net_do_receive(nc, buf, size);
+    }
+}
+
  static NetClientInfo net_virtio_info = {
      .type = NET_CLIENT_OPTIONS_KIND_NIC,
      .size = sizeof(NICState),
@@ -1814,6 +2304,7 @@ static void
virtio_net_device_realize(DeviceState *dev, Error **errp)
      nc = qemu_get_queue(n->nic);
      nc->rxfilter_notify_enabled = 1;
+    QTAILQ_INIT(&n->rsc_chains);
      n->qdev = dev;
      register_savevm(dev, "virtio-net", -1, VIRTIO_NET_VM_VERSION,
                      virtio_net_save, virtio_net_load, n);
@@ -1848,6 +2339,7 @@ static void
virtio_net_device_unrealize(DeviceState *dev, Error **errp)
      g_free(n->vqs);
      qemu_del_nic(n->nic);
      virtio_cleanup(vdev);
+    virtio_net_rsc_cleanup(n);
  }
  static void virtio_net_instance_init(Object *obj)
@@ -1909,6 +2401,10 @@ static Property virtio_net_properties[] = {
                         TX_TIMER_INTERVAL),
      DEFINE_PROP_INT32("x-txburst", VirtIONet, net_conf.txburst,
TX_BURST),
      DEFINE_PROP_STRING("tx", VirtIONet, net_conf.tx),
+    DEFINE_PROP_BIT("guest_rsc", VirtIONet, host_features,
+                    VIRTIO_NET_F_GUEST_RSC, false),
+    DEFINE_PROP_UINT32("rsc_interval", VirtIONet, rsc_timeout,
+                      VIRTIO_NET_RSC_INTERVAL),
      DEFINE_PROP_END_OF_LIST(),
  };
diff --git a/include/hw/virtio/virtio-net.h
b/include/hw/virtio/virtio-net.h
index 0cabdb6..e995190 100644
--- a/include/hw/virtio/virtio-net.h
+++ b/include/hw/virtio/virtio-net.h
@@ -59,6 +59,8 @@ typedef struct VirtIONet {
      VirtIONetQueue *vqs;
      VirtQueue *ctrl_vq;
      NICState *nic;
+    QTAILQ_HEAD(, NetRscChain) rsc_chains;
+    uint32_t rsc_timeout;
      uint32_t tx_timeout;
      int32_t tx_burst;
      uint32_t has_vnet_hdr;
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 6a37065..a7cb84a 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -30,6 +30,8 @@
                                  (0x1ULL << VIRTIO_F_ANY_LAYOUT))
  struct VirtQueue;
+struct VirtIONet;
+typedef struct VirtIONet VirtIONet;
  static inline hwaddr vring_align(hwaddr addr,
                                               unsigned long align)
@@ -128,6 +130,79 @@ typedef struct VirtioDeviceClass {
      int (*load)(VirtIODevice *vdev, QEMUFile *f, int version_id);
  } VirtioDeviceClass;
+/* Coalesced packets type & status */
+typedef enum {
+    RSC_COALESCE,           /* Data been coalesced */
+    RSC_FINAL,              /* Will terminate current connection */
+    RSC_NO_MATCH,           /* No matched in the buffer pool */
+    RSC_BYPASS,             /* Packet to be bypass, not tcp, tcp
ctrl, etc */
+    RSC_WANT                /* Data want to be coalesced */
+} COALESCE_STATUS;
+
+typedef struct NetRscStat {
+    uint32_t received;
+    uint32_t coalesced;
+    uint32_t over_size;
+    uint32_t cache;
+    uint32_t empty_cache;
+    uint32_t no_match_cache;
+    uint32_t win_update;
+    uint32_t no_match;
+    uint32_t tcp_syn;
+    uint32_t tcp_ctrl_drain;
+    uint32_t dup_ack;
+    uint32_t dup_ack1;
+    uint32_t dup_ack2;
+    uint32_t pure_ack;
+    uint32_t ack_out_of_win;
+    uint32_t data_out_of_win;
+    uint32_t data_out_of_order;
+    uint32_t data_after_pure_ack;
+    uint32_t bypass_not_tcp;
+    uint32_t tcp_option;
+    uint32_t tcp_all_opt;
+    uint32_t ip_frag;
+    uint32_t ip_hacked;
+    uint32_t ip_option;
+    uint32_t purge_failed;
+    uint32_t drain_failed;
+    uint32_t final_failed;
+    int64_t  timer;
+} NetRscStat;
+
+/* Rsc unit general info used to checking if can coalescing */
+typedef struct NetRscUnit {
+    void *ip;   /* ip header */
+    uint16_t *ip_plen;      /* data len pointer in ip header field */
+    struct tcp_header *tcp; /* tcp header */
+    uint16_t tcp_hdrlen;    /* tcp header len */
+    uint16_t payload;       /* pure payload without virtio/eth/ip/tcp */
+} NetRscUnit;
+
+/* Coalesced segmant */
+typedef struct NetRscSeg {
+    QTAILQ_ENTRY(NetRscSeg) next;
+    void *buf;
+    size_t size;
+    uint16_t packets;
+    uint16_t dup_ack;
+    bool is_coalesced;      /* need recal ipv4 header checksum, mark
here */
+    NetRscUnit unit;
+    NetClientState *nc;
+} NetRscSeg;
+
+/* Chain is divided by protocol(ipv4/v6) and NetClientInfo */
+typedef struct NetRscChain {
+    QTAILQ_ENTRY(NetRscChain) next;
+    VirtIONet *n;                            /* VirtIONet */
+    uint16_t proto;
+    uint8_t  gso_type;
+    uint16_t max_payload;
+    QEMUTimer *drain_timer;
+    QTAILQ_HEAD(, NetRscSeg) buffers;
+    NetRscStat stat;
+} NetRscChain;
+
  void virtio_instance_init_common(Object *proxy_obj, void *data,
                                   size_t vdev_size, const char
*vdev_name);
diff --git a/include/standard-headers/linux/virtio_net.h
b/include/standard-headers/linux/virtio_net.h
index a78f33e..5b95762 100644
--- a/include/standard-headers/linux/virtio_net.h
+++ b/include/standard-headers/linux/virtio_net.h
@@ -55,6 +55,7 @@
  #define VIRTIO_NET_F_MQ    22    /* Device supports Receive Flow
                       * Steering */
  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23    /* Set MAC address */
+#define VIRTIO_NET_F_GUEST_RSC  24      /* Guest can coalesce tcp
packets */

The comment should be "Guest can receive coalesced tcp packets" ?
Yes, actually windows driver is using gso_type 'GSO_TCPV4/6' for coalesced packets, i'm doubting if they are overlapped and can be multiplexed.

  #ifndef VIRTIO_NET_NO_LEGACY
  #define VIRTIO_NET_F_GSO    6    /* Host handles pkts w/ any GSO
type */




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