Re: [PATCH v2] libxl: Fix stubdom PCI passthrough

[Jan Beulich <jbeulich@xxxxxxxx>]

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