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.
Qemu smartcard/spicevmc support exists on branches (such as
http://cgit.freedesktop.org/~alon/qemu/commit/?h=usb_ccid.v15&id=024a37b)
but is not yet upstream. The added -help output matches a scratch build
that will be close to the RHEL 6.1 qemu-kvm.
* src/qemu/qemu_capabilities.h (QEMUD_CMD_FLAG_CCID_EMULATED)
(QEMUD_CMD_FLAG_CCID_PASSTHRU, QEMUD_CMD_FLAG_CHARDEV_SPICEVMC):
New flags.
* src/qemu/qemu_capabilities.c (qemuCapsComputeCmdFlags)
(qemuCapsParseDeviceStr): Check for smartcard capabilities.
(qemuCapsExtractVersionInfo): Tweak comment.
* tests/qemuhelptest.c (mymain): New test.
* tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel61: New file.
* tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel61-device: Likewise.
* src/qemu/qemu_capabilities.h (qemuCapsParseDeviceStr): New
prototype.
* src/qemu/qemu_capabilities.c (qemuCapsParsePCIDeviceStrs)
Rename and split...
(qemuCapsExtractDeviceStr, qemuCapsParseDeviceStr): ...to make it
easier to add and test device-specific checks.
(qemuCapsExtractVersionInfo): Update caller.
* tests/qemuhelptest.c (testHelpStrParsing): Also test parsing of
device-related flags.
(mymain): Update expected flags.
* tests/qemuhelpdata/qemu-0.12.1-device: New file.
* tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel60-device: New file.
* tests/qemuhelpdata/qemu-kvm-0.12.3-device: New file.
* tests/qemuhelpdata/qemu-kvm-0.13.0-device: New file.
* tests/qemuhelpdata/qemu-kvm-0.13.0: New file.
* tests/qemuhelptest.c (mymain): New test from Fedora 14 qemu-kvm,
which covers some options (like -fstype passthrough) not tested elsewhere.
* tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel60: New file.
* tests/qemuhelptest.c (mymain): New test from RHEL 6.0 qemu-kvm,
which covers some options (like -vga=qxl) not tested elsewhere.
* .x-sc_prohibit_empty_lines_at_EOF: Exempt qemu help output.
So far, readonly=on option is used when qemu supports -device. However,
there are qemu versions which support readonly option with -drive
although they don't have support for -device.
(gdb) p/x QEMUD_CMD_FLAG_VNET_HOST
$7 = 0xffffffff80000000
Oops - that meant we were incorrectly setting QEMU_CMD_FLAG_RTC_TD_HACK
for qemu-kvm-0.12.3 (and probably botching a few other settings as well).
Fixes Red Hat BZ#592070
* src/qemu/qemu_conf.h (QEMUD_CMD_FLAG_VNET_HOST): Avoid sign
extension.
* tests/qemuhelpdata/qemu-kvm-0.12.3: New file.
* tests/qemuhelptest.c (mymain): Add another case.
Probe for the new -device flag and if available set the -nodefaults
flag, instead of using -net none, -serial none or -parallel none.
Other device types will be converted to use -device in later patches.
The -nodefaults flag will help avoid unwelcome surprises from future
QEMU releases
* src/qemu/qemu_conf.c: Probe for -device. Add -nodefaults flag.
Remove -net none, -serial none or -parallel none
* src/qemu/qemu_conf.h: Define QEMU_CMD_FLAG_DEVICE
* tests/qemuhelpdata/qemu-0.12.1: New data file for 0.12.1 QEMU
* tests/qemuhelptest.c: Test feature extraction from 0.12.1 QEMU
