On 12.08.2021 02:57, Jason Andryuk wrote: > commit 0fdb48ffe7a1 "libxl: Make sure devices added by pci-attach are > reflected in the config" broken stubdom PCI passthrough when it moved > libxl__create_pci_backend later in the function. xl pci-attach for a > running PV domain may also have been broken, but that was not verified. > > The stubdomain is running (!starting) and PV, so it calls > libxl__wait_for_backend. With the new placement of > libxl__create_pci_backend, the path does not exist and the call > immediately fails. > libxl: error: libxl_device.c:1388:libxl__wait_for_backend: Backend > /local/domain/0/backend/pci/43/0 does not exist > libxl: error: libxl_pci.c:1764:device_pci_add_done: Domain > 42:libxl__device_pci_add failed for PCI device 0:2:0.0 (rc -3) > libxl: error: libxl_create.c:1857:domcreate_attach_devices: Domain 42:unable > to add pci devices > > The wait is only relevant when the backend is already present. num_devs > is already used to determine if the backend needs to be created. Re-use > num_devs to determine if the backend wait is necessary. The wait is > necessary to avoid racing with another PCI attachment reconfiguring the > front/back. If we are creating the backend, then we don't have to worry > about a racing reconfigure.
And why is such a race possible in the first place? If two independent actions are permitted in parallel on a domain, wouldn't there better be synchronization closer to the root of the call tree? Jan
