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

(Eduardo - I Cc'ed you because there's a question you may be able to answer about halfway down the message...) On 06/15/2017 10:17 AM, sferdjao(a)redhat.com wrote: I guess this is the upstream commit ID of the feature in qemu? I haven't noticed people referencing qemu commit ID's in libvirt in the past, but there's nothing wrong with it either. If you're going to put it in though, you should make it clear what the number is, maybe something like: Detect support for vhost-net busy polling (the "poll-us" commandline option). This was added to upstream qemu in version 2.7.0, commit 69e87b32.... It's not 100% done in existing code, but generally people have tried to reproduce qemu option names exactly in the capability name (I think less so in the past, more so now). So this should be vhost-net.poll-us (dash instead of underscore), or even more exactly "netdev.poll-us" or "netdev.tap.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. As implied up above, I think the capability should be called QEMU_CAPS_NETDEV_POLL_US, since that's an exact description of that goes on the commandline (and even though it currently only has an effect when vhost-net is turned on, that could possibly change in the future).