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[libvirt] [PATCHv3 2/2] Add txmode attribute to interface XML for virtio backend



==============================
V3 changes:

0) The actual code remains untouched from V2.

1) Add txmode attribute to the domain schema, and to an xml2xml and
   xml2argv test (note that the particular file used for the xml2xml
   test was previously only being used for xml2argv, and it needed
   some cleaning up before it would work properly.

2) Document the attribute on the website, along with a bold warning to
   leave it alone!
==============================
This is in response to:

   https://bugzilla.redhat.com/show_bug.cgi?id=629662

Explanation

qemu's virtio-net-pci driver allows setting the algorithm used for tx
packets to either "bh" or "timer". This is done by adding ",tx=bh" or
",tx=timer" to the "-device virtio-net-pci" commandline option.

'bh' stands for 'bottom half'; when this is set, packet tx is all done
in an iothread in the bottom half of the driver. (In libvirt, this
option is called the more descriptive "iothread".)

'timer' means that tx work is done in qemu, and if there is more tx
data than can be sent at the present time, a timer is set before qemu
moves on to do other things; when the timer fires, another attempt is
made to send more data. (libvirt retains the name "timer" for this
option.)

The resulting difference, according to the qemu developer who added
the option is:

    bh makes tx more asynchronous and reduces latency, but potentially
    causes more processor bandwidth contention since the cpu doing the
    tx isn't necessarily the cpu where the guest generated the
    packets.

Solution

This patch provides a libvirt domain xml knob to change the option on
the qemu commandline, by adding a new attribute "txmode" to the
<driver> element that can be placed inside any <interface> element in
a domain definition. It's use would be something like this:

    <interface ...>
      ...
      <model type='virtio'/>
      <driver txmode='iothread'/>
      ...
    </interface>

I chose to put this setting as an attribute to <driver> rather than as
a sub-element to <tune> because it is specific to the virtio-net
driver, not something that is generally usable by all network drivers.
(note that this is the same placement as the "driver name=..."
attribute used to choose kernel vs. userland backend for the
virtio-net driver.)

Actually adding the tx=xxx option to the qemu commandline is only done
if the version of qemu being used advertises it in the output of

    qemu -device virtio-net-pci,?

If a particular txmode is requested in the XML, and the option isn't
listed in that help output, an UNSUPPORTED_CONFIG error is logged, and
the domain fails to start.

---
 docs/formatdomain.html.in                          |   50 ++++++++++++++++++++
 docs/schemas/domain.rng                            |    8 +++
 src/conf/domain_conf.c                             |   26 ++++++++++-
 src/conf/domain_conf.h                             |   11 ++++
 src/qemu/qemu_capabilities.c                       |    3 +
 src/qemu/qemu_capabilities.h                       |    1 +
 src/qemu/qemu_command.c                            |   27 +++++++++++
 tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel61-device |   25 ++++++++++
 tests/qemuhelptest.c                               |    3 +-
 .../qemuxml2argv-net-virtio-device.args            |    2 +-
 .../qemuxml2argv-net-virtio-device.xml             |    3 +
 tests/qemuxml2argvtest.c                           |    2 +-
 tests/qemuxml2xmltest.c                            |    1 +
 13 files changed, 158 insertions(+), 4 deletions(-)

diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 9f7a113..84b1cab 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -1378,6 +1378,56 @@ qemu-kvm -net nic,model=? /dev/null
       ne2k_isa i82551 i82557b i82559er ne2k_pci pcnet rtl8139 e1000 virtio
     </p>
 
+    <h5><a name="elementsDriverBackendOptions">Setting NIC driver-specific options</a></h5>
+
+<pre>
+  ...
+  &lt;devices&gt;
+    &lt;interface type='network'&gt;
+      &lt;source network='default'/&gt;
+      &lt;target dev='vnet1'/&gt;
+      &lt;model type='virtio'/&gt;
+      <b>&lt;driver txmode='iothread'/&gt;</b>
+    &lt;/interface&gt;
+  &lt;/devices&gt;
+  ...</pre>
+
+    <p>
+      Some NICs may have tunable driver-specific options. These are
+      set as attributes of the <code>driver</code> sub-element of the
+      interface definition. Currently the following attributes are
+      available for the <code>"virtio"</code> NIC driver:
+    </p>
+
+    <dl>
+      <dt><code>txmode</code></dt>
+      <dd>
+        The <code>txmode</code> attribute specifies how to handle
+        transmission of packets when the transmit buffer is full. The
+        value can be either 'iothread' or 'timer'.
+        <span class="since">Since 0.8.8 (QEMU and KVM only)</span><br><br>
+
+        If set to 'iothread', packet tx is all done in an iothread in
+        the bottom half of the driver (this option translates into
+        adding "tx=bh" to the qemu commandline -device virtio-net-pci
+        option).<br><br>
+
+        If set to 'timer', tx work is done in qemu, and if there is
+        more tx data than can be sent at the present time, a timer is
+        set before qemu moves on to do other things; when the timer
+        fires, another attempt is made to send more data.<br><br>
+
+        The resulting difference, according to the qemu developer who
+        added the option is: "bh makes tx more asynchronous and reduces
+        latency, but potentially causes more processor bandwidth
+        contention since the cpu doing the tx isn't necessarily the
+        cpu where the guest generated the packets."<br><br>
+
+        <b>In general you should leave this option alone, unless you
+        are very certain you know what you are doing.</b>
+      </dd>
+    </dl>
+
     <h5><a name="elementsNICSTargetOverride">Overriding the target element</a></h5>
 
 <pre>
diff --git a/docs/schemas/domain.rng b/docs/schemas/domain.rng
index 7fe66e3..8b215f3 100644
--- a/docs/schemas/domain.rng
+++ b/docs/schemas/domain.rng
@@ -1042,6 +1042,14 @@
               </choice>
             </attribute>
           </optional>
+          <optional>
+            <attribute name="txmode">
+              <choice>
+                <value>iothread</value>
+                <value>timer</value>
+              </choice>
+            </attribute>
+          </optional>
           <empty/>
         </element>
       </optional>
diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index a069508..2aeaf1c 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -198,6 +198,11 @@ VIR_ENUM_IMPL(virDomainNetBackend, VIR_DOMAIN_NET_BACKEND_TYPE_LAST,
               "qemu",
               "vhost")
 
+VIR_ENUM_IMPL(virDomainNetVirtioTxMode, VIR_DOMAIN_NET_VIRTIO_TX_MODE_LAST,
+              "default",
+              "iothread",
+              "timer")
+
 VIR_ENUM_IMPL(virDomainChrChannelTarget,
               VIR_DOMAIN_CHR_CHANNEL_TARGET_TYPE_LAST,
               "guestfwd",
@@ -2477,6 +2482,7 @@ virDomainNetDefParseXML(virCapsPtr caps,
     char *port = NULL;
     char *model = NULL;
     char *backend = NULL;
+    char *txmode = NULL;
     char *filter = NULL;
     char *internal = NULL;
     char *devaddr = NULL;
@@ -2565,6 +2571,7 @@ virDomainNetDefParseXML(virCapsPtr caps,
                 model = virXMLPropString(cur, "type");
             } else if (xmlStrEqual (cur->name, BAD_CAST "driver")) {
                 backend = virXMLPropString(cur, "name");
+                txmode = virXMLPropString(cur, "txmode");
             } else if (xmlStrEqual (cur->name, BAD_CAST "filterref")) {
                 filter = virXMLPropString(cur, "filter");
                 VIR_FREE(filterparams);
@@ -2769,6 +2776,18 @@ virDomainNetDefParseXML(virCapsPtr caps,
             }
             def->driver.virtio.name = name;
         }
+        if (txmode != NULL) {
+            int m;
+            if (((m = virDomainNetVirtioTxModeTypeFromString(txmode)) < 0) ||
+                (m == VIR_DOMAIN_NET_VIRTIO_TX_MODE_DEFAULT)) {
+                virDomainReportError(VIR_ERR_INTERNAL_ERROR,
+                                     _("Unknown interface <driver txmode='%s'> "
+                                       "has been specified"),
+                                     txmode);
+                goto error;
+            }
+            def->driver.virtio.txmode = m;
+        }
     }
 
     if (filter != NULL) {
@@ -2808,6 +2827,7 @@ cleanup:
     VIR_FREE(bridge);
     VIR_FREE(model);
     VIR_FREE(backend);
+    VIR_FREE(txmode);
     VIR_FREE(filter);
     VIR_FREE(type);
     VIR_FREE(internal);
@@ -6813,12 +6833,16 @@ virDomainNetDefFormat(virBufferPtr buf,
         virBufferEscapeString(buf, "      <model type='%s'/>\n",
                               def->model);
         if (STREQ(def->model, "virtio") &&
-            def->driver.virtio.name) {
+            (def->driver.virtio.name || def->driver.virtio.txmode)) {
             virBufferAddLit(buf, "      <driver");
             if (def->driver.virtio.name) {
                 virBufferVSprintf(buf, " name='%s'",
                                   virDomainNetBackendTypeToString(def->driver.virtio.name));
             }
+            if (def->driver.virtio.txmode) {
+                virBufferVSprintf(buf, " txmode='%s'",
+                                  virDomainNetVirtioTxModeTypeToString(def->driver.virtio.txmode));
+            }
             virBufferAddLit(buf, "/>\n");
         }
     }
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index e05064f..30aeccc 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -321,6 +321,15 @@ enum virDomainNetBackendType {
     VIR_DOMAIN_NET_BACKEND_TYPE_LAST,
 };
 
+/* the TX algorithm used for virtio interfaces */
+enum virDomainNetVirtioTxModeType {
+    VIR_DOMAIN_NET_VIRTIO_TX_MODE_DEFAULT, /* default for this version of qemu */
+    VIR_DOMAIN_NET_VIRTIO_TX_MODE_IOTHREAD,
+    VIR_DOMAIN_NET_VIRTIO_TX_MODE_TIMER,
+
+    VIR_DOMAIN_NET_VIRTIO_TX_MODE_LAST,
+};
+
 /* the mode type for macvtap devices */
 enum virDomainNetdevMacvtapType {
     VIR_DOMAIN_NETDEV_MACVTAP_MODE_VEPA,
@@ -341,6 +350,7 @@ struct _virDomainNetDef {
     union {
         struct {
             enum virDomainNetBackendType name; /* which driver backend to use */
+            enum virDomainNetVirtioTxModeType txmode;
         } virtio;
     } driver;
     union {
@@ -1371,6 +1381,7 @@ VIR_ENUM_DECL(virDomainFS)
 VIR_ENUM_DECL(virDomainFSAccessMode)
 VIR_ENUM_DECL(virDomainNet)
 VIR_ENUM_DECL(virDomainNetBackend)
+VIR_ENUM_DECL(virDomainNetVirtioTxMode)
 VIR_ENUM_DECL(virDomainChrDevice)
 VIR_ENUM_DECL(virDomainChrChannelTarget)
 VIR_ENUM_DECL(virDomainChrConsoleTarget)
diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index 4504b7e..bd43087 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -1072,6 +1072,7 @@ qemuCapsExtractDeviceStr(const char *qemu,
                                "-device", "?",
                                "-device", "pci-assign,?",
                                "-device", "virtio-blk-pci,?",
+                               "-device", "virtio-net-pci,?",
                                NULL);
     virCommandAddEnvPassCommon(cmd);
     /* qemu -help goes to stdout, but qemu -device ? goes to stderr.  */
@@ -1113,6 +1114,8 @@ qemuCapsParseDeviceStr(const char *str, unsigned long long *flags)
         if (strstr(str, "pci-assign.bootindex"))
             *flags |= QEMUD_CMD_FLAG_PCI_BOOTINDEX;
     }
+    if (strstr(str, "virtio-net-pci.tx="))
+        *flags |= QEMUD_CMD_FLAG_VIRTIO_TX_ALG;
 
     return 0;
 }
diff --git a/src/qemu/qemu_capabilities.h b/src/qemu/qemu_capabilities.h
index dd39b3b..c29d914 100644
--- a/src/qemu/qemu_capabilities.h
+++ b/src/qemu/qemu_capabilities.h
@@ -92,6 +92,7 @@ enum qemuCapsFlags {
     QEMUD_CMD_FLAG_CCID_PASSTHRU = (1LL << 55), /* -device ccid-card-passthru */
     QEMUD_CMD_FLAG_CHARDEV_SPICEVMC = (1LL << 56), /* newer -chardev spicevmc */
     QEMUD_CMD_FLAG_DEVICE_SPICEVMC = (1LL << 57), /* older -device spicevmc*/
+    QEMUD_CMD_FLAG_VIRTIO_TX_ALG = (1LL << 58), /* -device virtio-net-pci,tx=string */
 };
 
 virCapsPtr qemuCapsInit(virCapsPtr old_caps);
diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 1b213da..7b49b3c 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -1577,16 +1577,43 @@ qemuBuildNicDevStr(virDomainNetDefPtr net,
 {
     virBuffer buf = VIR_BUFFER_INITIALIZER;
     const char *nic;
+    bool usingVirtio = false;
 
     if (!net->model) {
         nic = "rtl8139";
     } else if (STREQ(net->model, "virtio")) {
         nic = "virtio-net-pci";
+        usingVirtio = true;
     } else {
         nic = net->model;
     }
 
     virBufferAdd(&buf, nic, strlen(nic));
+    if (usingVirtio && net->driver.virtio.txmode) {
+        if (qemuCmdFlags & QEMUD_CMD_FLAG_VIRTIO_TX_ALG) {
+            virBufferAddLit(&buf, ",tx=");
+            switch (net->driver.virtio.txmode) {
+                case VIR_DOMAIN_NET_VIRTIO_TX_MODE_IOTHREAD:
+                    virBufferAddLit(&buf, "bh");
+                    break;
+
+                case VIR_DOMAIN_NET_VIRTIO_TX_MODE_TIMER:
+                    virBufferAddLit(&buf, "timer");
+                    break;
+                default:
+                    /* this should never happen, if it does, we need
+                     * to add another case to this switch.
+                     */
+                    qemuReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                                    _("unrecognized virtio-net-pci 'tx' option"));
+                    goto error;
+            }
+        } else {
+            qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                            _("virtio-net-pci 'tx' option not supported in this QEMU binary"));
+            goto error;
+        }
+    }
     if (vlan == -1)
         virBufferVSprintf(&buf, ",netdev=host%s", net->info.alias);
     else
diff --git a/tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel61-device b/tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel61-device
index 8a0e528..8ac9630 100644
--- a/tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel61-device
+++ b/tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel61-device
@@ -72,3 +72,28 @@ virtio-blk-pci.ioeventfd=on/off
 virtio-blk-pci.vectors=uint32
 virtio-blk-pci.indirect_desc=on/off
 virtio-blk-pci.scsi=on/off
+virtio-net-pci.vectors=uint32
+virtio-net-pci.indirect_desc=on/off
+virtio-net-pci.csum=on/off
+virtio-net-pci.guest_csum=on/off
+virtio-net-pci.gso=on/off
+virtio-net-pci.guest_tso4=on/off
+virtio-net-pci.guest_tso6=on/off
+virtio-net-pci.guest_ecn=on/off
+virtio-net-pci.guest_ufo=on/off
+virtio-net-pci.host_tso4=on/off
+virtio-net-pci.host_tso6=on/off
+virtio-net-pci.host_ecn=on/off
+virtio-net-pci.host_ufo=on/off
+virtio-net-pci.mrg_rxbuf=on/off
+virtio-net-pci.status=on/off
+virtio-net-pci.ctrl_vq=on/off
+virtio-net-pci.ctrl_rx=on/off
+virtio-net-pci.ctrl_vlan=on/off
+virtio-net-pci.ctrl_rx_extra=on/off
+virtio-net-pci.mac=macaddr
+virtio-net-pci.vlan=vlan
+virtio-net-pci.netdev=netdev
+virtio-net-pci.x-txtimer=uint32
+virtio-net-pci.x-txburst=int32
+virtio-net-pci.tx=string
diff --git a/tests/qemuhelptest.c b/tests/qemuhelptest.c
index 3a04b61..11890ab 100644
--- a/tests/qemuhelptest.c
+++ b/tests/qemuhelptest.c
@@ -480,7 +480,8 @@ mymain(int argc, char **argv)
             QEMUD_CMD_FLAG_HDA_DUPLEX |
             QEMUD_CMD_FLAG_DRIVE_AIO |
             QEMUD_CMD_FLAG_CCID_PASSTHRU |
-            QEMUD_CMD_FLAG_CHARDEV_SPICEVMC,
+            QEMUD_CMD_FLAG_CHARDEV_SPICEVMC |
+            QEMUD_CMD_FLAG_VIRTIO_TX_ALG,
             12001, 1,  0);
 
     return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
diff --git a/tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.args b/tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.args
index 92bd889..843c3e9 100644
--- a/tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.args
+++ b/tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.args
@@ -1,6 +1,6 @@
 LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test /usr/bin/qemu -S -M \
 pc -m 214 -smp 1 -nographic -nodefconfig -nodefaults -monitor \
 unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -hda \
-/dev/HostVG/QEMUGuest1 -device virtio-net-pci,vlan=0,id=net0,\
+/dev/HostVG/QEMUGuest1 -device virtio-net-pci,tx=bh,vlan=0,id=net0,\
 mac=00:11:22:33:44:55,bus=pci.0,addr=0x2 -net user,vlan=0,name=hostnet0 -usb \
 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3
diff --git a/tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.xml b/tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.xml
index 572f3cc..4e7abcd 100644
--- a/tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.xml
+++ b/tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.xml
@@ -17,10 +17,13 @@
     <disk type='block' device='disk'>
       <source dev='/dev/HostVG/QEMUGuest1'/>
       <target dev='hda' bus='ide'/>
+      <address type='drive' controller='0' bus='0' unit='0'/>
     </disk>
+    <controller type='ide' index='0'/>
     <interface type='user'>
       <mac address='00:11:22:33:44:55'/>
       <model type='virtio'/>
+      <driver txmode='iothread'/>
     </interface>
     <memballoon model='virtio'/>
   </devices>
diff --git a/tests/qemuxml2argvtest.c b/tests/qemuxml2argvtest.c
index 52808b5..b42495f 100644
--- a/tests/qemuxml2argvtest.c
+++ b/tests/qemuxml2argvtest.c
@@ -356,7 +356,7 @@ mymain(int argc, char **argv)
     DO_TEST("net-user", 0, false);
     DO_TEST("net-virtio", 0, false);
     DO_TEST("net-virtio-device", QEMUD_CMD_FLAG_DEVICE |
-            QEMUD_CMD_FLAG_NODEFCONFIG, false);
+            QEMUD_CMD_FLAG_NODEFCONFIG | QEMUD_CMD_FLAG_VIRTIO_TX_ALG, false);
     DO_TEST("net-virtio-netdev", QEMUD_CMD_FLAG_DEVICE |
             QEMUD_CMD_FLAG_NETDEV | QEMUD_CMD_FLAG_NODEFCONFIG, false);
     DO_TEST("net-eth", 0, false);
diff --git a/tests/qemuxml2xmltest.c b/tests/qemuxml2xmltest.c
index 15d94b7..67e721b 100644
--- a/tests/qemuxml2xmltest.c
+++ b/tests/qemuxml2xmltest.c
@@ -159,6 +159,7 @@ mymain(int argc, char **argv)
     DO_TEST("misc-no-reboot");
     DO_TEST("net-user");
     DO_TEST("net-virtio");
+    DO_TEST("net-virtio-device");
     DO_TEST("net-eth");
     DO_TEST("net-eth-ifname");
     DO_TEST("sound");
-- 
1.7.3.4


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