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.
This patch teaches testutil how to read multi-line input files with
backspace-newline line continuation markers.
The patch also breaks up all the single-line arguments test input files into
multi-line files with lines shorter than 80 characters.
To try and ensure that people upgrading from old QEMU get guests
with the same PCI device ordering, change the way we assign addrs
to match QEMU's default order. This should make Windows less
annoyed.
* src/qemu/qemu_conf.c: Follow QEMU's default PCI ordering
logic when assigning addresses
* tests/*.args: Update for changed PCI addresses
We already use the '-nodefaults' command line arg with QEMU to stop
it adding any default devices to guests. Unfortunately, QEMU will
load global config files from /etc/qemu that may also add default
devices. These aren't blocked by '-nodefaults', so we need to also
add the '-nodefconfig' arg to prevent that.
Unfortunately these global config files are also used to define
custom CPU models. So in blocking global hardware device addition
we also block definitions of new CPU models. Libvirt doesn't know
about these custom CPU models though, so it would never make use
of them anyway. Thus blocking them via -nodefconfig isn't a show
stopping problem. We would need to expand libvirt's own CPU model
XML database to support these instead.
* src/qemu/qemu_conf.c: Add '-nodefconfig' if available
* tests/qemuxml2argvdata/: Add '-nodefconfig' to all data files which
have '-nodefaults' present
The current way of assigning names to the host network backend and
NIC device in QEMU was over complicated, by varying naming scheme
based on the NIC model and backend type. This simplifies the naming
to simply be 'net0' and 'hostnet0', allowing code to easily determine
the host network name and vlan based off the primary device alias
name 'net0'. This in turn allows removal of alot of QEMU specific
code from the XML parser, and makes it easier to assign new unique
names for NICs that are hotplugged
* src/conf/domain_conf.c, src/conf/domain_conf.h: Remove hostnet_name
and vlan fields from virNetworkDefPtr
* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h, src/qemu/qemu_driver.c:
Use a single network alias naming scheme regardless of NIC type
or backend type. Determine VLANs from the alias name.
* tests/qemuxml2argvdata/qemuxml2argv-net-eth-names.args,
tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.args,
tests/qemuxml2argvdata/qemuxml2argv-net-virtio-netdev.args: Update
for new simpler naming scheme
The QEMU 0.12.x tree has the -netdev command line argument, but not
corresponding monitor command. We can't enable the former, without
the latter since it will break hotplug/unplug.
* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Disable -netdev usage
until 0.13 at earliest
* tests/qemuxml2argvtest.c: Add test for -netdev syntax
* tests/qemuxml2argvdata/qemuxml2argv-net-virtio-netdev.args,
tests/qemuxml2argvdata/qemuxml2argv-net-virtio-netdev.xml: Test
data files for -netdev syntax
Replace
-balloon virtio
With
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3
This allows it to get correct assigned PCI address as declared in
previous patch
* src/qemu/qemu_conf.c: Convert Virtio ballon to -device and
give it an explicit PCI address
* tests/qemuxml2argvdata/qemuxml2argv-*args: Add in virtio balloon
where appropriate
Instead of relying on QEMU to assign PCI addresses and then querying
them with 'info pci', manually assign all PCI addresses before starting
the guest. These addresses are not stable across reboots. That will
come in a later patch
NB, the PIIX3 (IDE, FDC, ISA-Bridge) will always have slot 1 and
VGA will always have slot 2. We declare the Virtio Balloon gets
slot 3, and then all remaining slots are for configured devices.
* src/qemu/qemu_conf.c: If -device is supported, then assign all PCI
addresses when building the command line
* src/qemu/qemu_driver.c: Don't query monitor for PCI addresses if
they have already been assigned
* tests/qemuxml2argvdata/qemuxml2argv-hostdev-pci-address-device.args,
tests/qemuxml2argvdata/qemuxml2argv-net-virtio-device.args,
tests/qemuxml2argvdata/qemuxml2argv-sound-device.args,
tests/qemuxml2argvdata/qemuxml2argv-watchdog-device.args: Update
to include PCI slot/bus information
The current syntax uses a pair of args
-net nic,macaddr=52:54:00:56:6c:55,vlan=3,model=pcnet,name=pcnet.0
-net user,vlan=3,name=user.0
The new syntax does not need the vlan craziness anymore, and
so has a simplified pair of args
-netdev user,id=user.0
-device pcnet,netdev=user.0,id=pcnet.0,mac=52:54:00:56:6c:55,addr=<PCI SLOT>
