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

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