Re: [libvirt] [PATCH 1/3] qemu: add capability for vhost-net busy polling

> + /* Support for busy polling on vhost-net devices ("-netdev > + * tap,...,poll-us=n") */ > + if (qemuCaps->version >= 2007000) > + virQEMUCapsSet(qemuCaps, QEMU_CAPS_VHOST_NET_POLL_US); > + Enabling a capability based purely on the qemu binary version is something that's only done as a last resort if it can't be determined by QMP probes. This is because the version number is an inaccurate indicator - it often happens that a new capability is added to upstream in qemu version "Y", but then that feature is backported to qemu version "X" in some downstream distro package; if we based the capability on version number, then libvirt wouldn't take advantage of the feature even though qemu had it. What *should* get us the info we need is to add a table entry to virQEMUCapsCommandLine to check whether or not the poll-us option exists in the current qemu binary. Something like this: static struct virQEMUCapsCommandLineProps virQEMUCapsCommandLine[] = { ... { "netdev", "poll-us", QEMU_CAPS_NETDEV_POLL_US }, Unfortunately when I add that and regenerate the .replies file for qemu 2.9.0 (with the command "tests/qemucapsprobe /usr/bin/qemu-system-x86_64"), this is the info it finds for netdev: { "parameters": [ ], "option": "netdev" }, My understanding was that the possible parameters to the netdev commandline option should be listed, but instead the parameters list is empty :-/. Maybe Eduardo can help me understand why the parameters are empty, and (more importantly) what needs to be done to get the supported parameters for -netdev - basically we need to know what are the available parameters to "-netdev tap,...". I hope we don't end up needing to base the check purely on qemu version.