The following commit:

  1fb3a8b2 ("xen/spinlock: Fix locking path engaging too soon under PVHVM.")

... moved the initalization of the kicker interrupt until after
native_cpu_up() is called.

However, when using qspinlocks, a CPU may try to kick another CPU that is
spinning (because it has not yet initialized its kicker interrupt), resulting
in the following crash during boot:

  kernel BUG at /build/linux-Ay7j_C/linux-4.4.0/drivers/xen/events/events_base.c:1210!
  invalid opcode: 0000 [#1] SMP
  ...
  RIP: 0010:[<ffffffff814c97c9>]  [<ffffffff814c97c9>] xen_send_IPI_one+0x59/0x60
  ...
  Call Trace:
   [<ffffffff8102be9e>] xen_qlock_kick+0xe/0x10
   [<ffffffff810cabc2>] __pv_queued_spin_unlock+0xb2/0xf0
   [<ffffffff810ca6d1>] ? __raw_callee_save___pv_queued_spin_unlock+0x11/0x20
   [<ffffffff81052936>] ? check_tsc_warp+0x76/0x150
   [<ffffffff81052aa6>] check_tsc_sync_source+0x96/0x160
   [<ffffffff81051e28>] native_cpu_up+0x3d8/0x9f0
   [<ffffffff8102b315>] xen_hvm_cpu_up+0x35/0x80
   [<ffffffff8108198c>] _cpu_up+0x13c/0x180
   [<ffffffff81081a4a>] cpu_up+0x7a/0xa0
   [<ffffffff81f80dfc>] smp_init+0x7f/0x81
   [<ffffffff81f5a121>] kernel_init_freeable+0xef/0x212
   [<ffffffff81817f30>] ? rest_init+0x80/0x80
   [<ffffffff81817f3e>] kernel_init+0xe/0xe0
   [<ffffffff8182488f>] ret_from_fork+0x3f/0x70
   [<ffffffff81817f30>] ? rest_init+0x80/0x80

To fix this, only send the kick if the target CPU's interrupt has been
initialized. This check isn't racy, because the target is waiting for
the spinlock, so it won't have initialized the interrupt in the
meantime.
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: xen-devel@lists.xenproject.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>