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

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.