[PATCH 2/2] xen/virtio: Avoid use of the dom0 backend in dom0

[Petr Pavlu <petr.pavlu@xxxxxxxx>]

When attempting to run Xen on a QEMU/KVM virtual machine with virtio
devices (all x86_64), dom0 tries to establish a grant for itself which
eventually results in a hang during the boot.

The backtrace looks as follows, the while loop in __send_control_msg()
makes no progress:

  #0  virtqueue_get_buf_ctx (_vq=_vq@entry=0xffff8880074a8400, 
len=len@entry=0xffffc90000413c94, ctx=ctx@entry=0x0 <fixed_percpu_data>) at 
../drivers/virtio/virtio_ring.c:2326
  #1  0xffffffff817086b7 in virtqueue_get_buf 
(_vq=_vq@entry=0xffff8880074a8400, len=len@entry=0xffffc90000413c94) at 
../drivers/virtio/virtio_ring.c:2333
  #2  0xffffffff8175f6b2 in __send_control_msg (portdev=<optimized out>, 
port_id=0xffffffff, event=0x0, value=0x1) at 
../drivers/char/virtio_console.c:562
  #3  0xffffffff8175f6ee in __send_control_msg (portdev=<optimized out>, 
port_id=<optimized out>, event=<optimized out>, value=<optimized out>) at 
../drivers/char/virtio_console.c:569
  #4  0xffffffff817618b1 in virtcons_probe (vdev=0xffff88800585e800) at 
../drivers/char/virtio_console.c:2098
  #5  0xffffffff81707117 in virtio_dev_probe (_d=0xffff88800585e810) at 
../drivers/virtio/virtio.c:305
  #6  0xffffffff8198e348 in call_driver_probe (drv=0xffffffff82be40c0 
<virtio_console>, drv=0xffffffff82be40c0 <virtio_console>, 
dev=0xffff88800585e810) at ../drivers/base/dd.c:579
  #7  really_probe (dev=dev@entry=0xffff88800585e810, 
drv=drv@entry=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/dd.c:658
  #8  0xffffffff8198e58f in __driver_probe_device 
(drv=drv@entry=0xffffffff82be40c0 <virtio_console>, 
dev=dev@entry=0xffff88800585e810) at ../drivers/base/dd.c:800
  #9  0xffffffff8198e65a in driver_probe_device 
(drv=drv@entry=0xffffffff82be40c0 <virtio_console>, 
dev=dev@entry=0xffff88800585e810) at ../drivers/base/dd.c:830
  #10 0xffffffff8198e832 in __driver_attach (dev=0xffff88800585e810, 
data=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/dd.c:1216
  #11 0xffffffff8198bfb2 in bus_for_each_dev (bus=<optimized out>, 
start=start@entry=0x0 <fixed_percpu_data>, data=data@entry=0xffffffff82be40c0 
<virtio_console>,
      fn=fn@entry=0xffffffff8198e7b0 <__driver_attach>) at 
../drivers/base/bus.c:368
  #12 0xffffffff8198db65 in driver_attach (drv=drv@entry=0xffffffff82be40c0 
<virtio_console>) at ../drivers/base/dd.c:1233
  #13 0xffffffff8198d207 in bus_add_driver (drv=drv@entry=0xffffffff82be40c0 
<virtio_console>) at ../drivers/base/bus.c:673
  #14 0xffffffff8198f550 in driver_register (drv=drv@entry=0xffffffff82be40c0 
<virtio_console>) at ../drivers/base/driver.c:246
  #15 0xffffffff81706b47 in register_virtio_driver 
(driver=driver@entry=0xffffffff82be40c0 <virtio_console>) at 
../drivers/virtio/virtio.c:357
  #16 0xffffffff832cd34b in virtio_console_init () at 
../drivers/char/virtio_console.c:2258
  #17 0xffffffff8100105c in do_one_initcall (fn=0xffffffff832cd2e0 
<virtio_console_init>) at ../init/main.c:1246
  #18 0xffffffff83277293 in do_initcall_level (command_line=0xffff888003e2f900 
"root", level=0x6) at ../init/main.c:1319
  #19 do_initcalls () at ../init/main.c:1335
  #20 do_basic_setup () at ../init/main.c:1354
  #21 kernel_init_freeable () at ../init/main.c:1571
  #22 0xffffffff81f64be1 in kernel_init (unused=<optimized out>) at 
../init/main.c:1462
  #23 0xffffffff81001f49 in ret_from_fork () at ../arch/x86/entry/entry_64.S:308
  #24 0x0000000000000000 in ?? ()

Fix the problem by preventing xen_grant_init_backend_domid() from
setting dom0 as a backend when running in dom0.

Fixes: 035e3a4321f7 ("xen/virtio: Optimize the setup of "xen-grant-dma" 
devices")
Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
---
 drivers/xen/grant-dma-ops.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
index 76f6f26265a3..29ed27ac450e 100644
--- a/drivers/xen/grant-dma-ops.c
+++ b/drivers/xen/grant-dma-ops.c
@@ -362,7 +362,9 @@ static int xen_grant_init_backend_domid(struct device *dev,
        if (np) {
                ret = xen_dt_grant_init_backend_domid(dev, np, backend_domid);
                of_node_put(np);
-       } else if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || 
xen_pv_domain()) {
+       } else if ((IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) ||
+                   xen_pv_domain()) &&
+                  !xen_initial_domain()) {
                dev_info(dev, "Using dom0 as backend\n");
                *backend_domid = 0;
                ret = 0;
-- 
2.35.3