Problems starting Xen domU after latest stable update

[Michael D Labriola <michael.d.labriola@gmail.com>]

Hey, everyone.  I've run into problems starting up my Xen domUs as of
the latest batch of stable updates.  Whenever I try to create one, I
get a bunch of block device errors like this:

libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to add device with path /local/domain/0/backend/vbd/4/51712
libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to add device with path /local/domain/0/backend/vbd/4/51728
libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to add device with path /local/domain/0/backend/vbd/4/51744
libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to add device with path /local/domain/0/backend/vbd/4/51760
libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to add device with path /local/domain/0/backend/vbd/4/51776
libxl: error: libxl_create.c:1452:domcreate_launch_dm: Domain 4:unable to add disk devices
libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to remove device with path /local/domain/0/backend/vbd/4/51712
libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to remove device with path /local/domain/0/backend/vbd/4/51728
libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to remove device with path /local/domain/0/backend/vbd/4/51744
libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to remove device with path /local/domain/0/backend/vbd/4/51760
libxl: error: libxl_device.c:1105:device_backend_callback: Domain 4:unable to remove device with path /local/domain/0/backend/vbd/4/51776
libxl: error: libxl_domain.c:1290:devices_destroy_cb: Domain 4:libxl__devices_destroy failed
libxl: error: libxl_domain.c:1177:libxl__destroy_domid: Domain 4:Non-existant domain
libxl: error: libxl_domain.c:1131:domain_destroy_callback: Domain 4:Unable to destroy guest
libxl: error: libxl_domain.c:1058:domain_destroy_cb: Domain 4:Destruction of domain failed

I'm using Xen 4.13.1 on the box I've been testing with.

I bisected down to this commit, and reverting it does indeed fix my
problem.  Well, this commit upstream and it's cherry-picked variants
on linux-5.4.y and linux-5.10.y.

commit 3499ba8198cad47b731792e5e56b9ec2a78a83a2
Author: David Woodhouse <dwmw@amazon.co.uk>
Date:   Wed Jan 13 13:26:02 2021 +0000

    xen: Fix event channel callback via INTX/GSI
    
    For a while, event channel notification via the PCI platform device
    has been broken, because we attempt to communicate with xenstore before
    we even have notifications working, with the xs_reset_watches() call
    in xs_init().
    
    We tend to get away with this on Xen versions below 4.0 because we avoid
    calling xs_reset_watches() anyway, because xenstore might not cope with
    reading a non-existent key. And newer Xen *does* have the vector
    callback support, so we rarely fall back to INTX/GSI delivery.
    
    To fix it, clean up a bit of the mess of xs_init() and xenbus_probe()
    startup. Call xs_init() directly from xenbus_init() only in the !XS_HVM
    case, deferring it to be called from xenbus_probe() in the XS_HVM case
    instead.
    
    Then fix up the invocation of xenbus_probe() to happen either from its
    device_initcall if the callback is available early enough, or when the
    callback is finally set up. This means that the hack of calling
    xenbus_probe() from a workqueue after the first interrupt, or directly
    from the PCI platform device setup, is no longer needed.
    
    Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
    Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Link: https://lore.kernel.org/r/20210113132606.422794-2-dwmw2@infradead.org
    Signed-off-by: Juergen Gross <jgross@suse.com>