Re: [libvirt] [RFC] On present using dummy hostdev usb device

On Fri, Aug 30, 2019 at 05:08:52PM +0200, Michal Privoznik wrote: > On 8/30/19 2:30 PM, Roman Kagan wrote: >> On Fri, Aug 30, 2019 at 10:09:06AM +0100, Daniel P. Berrangé wrote: >>> On Fri, Aug 30, 2019 at 08:44:03AM +0000, Nikolay Shirokovskiy wrote: >>>> Hi, all! >>>> >>>> We use an interesting approach when starting/migrating/etc domain with usb >>>> hostdev with startupPolicy=optional. We add qemu usb-host device with >>>> missing hostaddr/hostbus parameters (dummy device). I guess there are >>>> 2 reasons why we do it. First without dummy device migration will fail as >>>> described in [1]. Second is an interesting property of dummy device that >>>> qemu starts to monitor for attaching of usb devices and binds the first >>>> attached to node to the dummy device. So one can start a domain with >>>> missing hostdev and attach it later or migrate a domain then detach >>>> hostdev on source and attach it on destination. But as qemu binds the >>>> first attached device this is not reliable, to say the least. And after >>>> all this does not work if domain uses distinct mount namespace which >>>> is default. >>> >>> Even without mount namespaces, it should fail as QEMU is running non-root >>> and libvirt won't have granted access to any host USB devices in /dev, and >>> also SELinux policy will forbid this. >> >> Right, but the case with mount namespaces is particularly problematic: >> if the device open fails due to missing device node, libusb removes the >> device from its internal device list. This results in the following >> scenario: >> >> - libvirt adds a dummy usb-host device to QEMU in place of a missing >> device >> >> - QEMU (via libusb) installs a watch for udev add events >> >> - the physical device is plugged into the host >> >> - QEMU detects the addition of the device and, since the dummy device >> matches everything, tries to open it >> >> - by this time libvirt may have not created a device node in QEMU's >> mount namespace, so the open fails due to missing device node, and >> libusb removes the device from its internal list >> >> - libvirt removes the dummy usb-host device and adds the actual usb-host >> device >> >> - QEMU fails to open it because it's no longer seen by libusb > > There is a bug filed against libusb exactly for this: > > https://bugzilla.redhat.com/show_bug.cgi?id=1595525 > > BTW: you don't have to migrate, it's sufficient to start a domain with a > missing USB and startupPolicy='optional' and then physically plug it into > the host and then try to hotplug it into the domain. AFAIR, the startupPolicy=optional was never really intended to allow for the device to be dynamically added after startup. It was only trying drop the device if it didn't exist at startup.