The user has the option of disabling the platform driver:
00:02.0 Unassigned class [ff80]: XenSource, Inc. Xen Platform Device (rev 01)

which is used to unplug the emulated drivers (IDE, Realtek 8169, etc)
and allow the PV drivers to take over. If the user wishes
to disable that they can set:

  xen_platform_pci=0
  (in the guest config file)

or
  xen_emul_unplug=never
  (on the Linux command line)

except it does not work properly. The PV drivers still try to
load and since the Xen platform driver is not run - and it
has not initialized the grant tables, most of the PV drivers
stumble upon:

input: Xen Virtual Keyboard as /devices/virtual/input/input5
input: Xen Virtual Pointer as /devices/virtual/input/input6M
------------[ cut here ]------------
kernel BUG at /home/konrad/ssd/konrad/linux/drivers/xen/grant-table.c:1206!
invalid opcode: 0000 [#1] SMP
Modules linked in: xen_kbdfront(+) xenfs xen_privcmd
CPU: 6 PID: 1389 Comm: modprobe Not tainted 3.13.0-rc1upstream-00021-ga6c892b-dirty #1
Hardware name: Xen HVM domU, BIOS 4.4-unstable 11/26/2013
RIP: 0010:[<ffffffff813ddc40>]  [<ffffffff813ddc40>] get_free_entries+0x2e0/0x300
Call Trace:
 [<ffffffff8150d9a3>] ? evdev_connect+0x1e3/0x240
 [<ffffffff813ddd0e>] gnttab_grant_foreign_access+0x2e/0x70
 [<ffffffffa0010081>] xenkbd_connect_backend+0x41/0x290 [xen_kbdfront]
 [<ffffffffa0010a12>] xenkbd_probe+0x2f2/0x324 [xen_kbdfront]
 [<ffffffff813e5757>] xenbus_dev_probe+0x77/0x130
 [<ffffffff813e7217>] xenbus_frontend_dev_probe+0x47/0x50
 [<ffffffff8145e9a9>] driver_probe_device+0x89/0x230
 [<ffffffff8145ebeb>] __driver_attach+0x9b/0xa0
 [<ffffffff8145eb50>] ? driver_probe_device+0x230/0x230
 [<ffffffff8145eb50>] ? driver_probe_device+0x230/0x230
 [<ffffffff8145cf1c>] bus_for_each_dev+0x8c/0xb0
 [<ffffffff8145e7d9>] driver_attach+0x19/0x20
 [<ffffffff8145e260>] bus_add_driver+0x1a0/0x220
 [<ffffffff8145f1ff>] driver_register+0x5f/0xf0
 [<ffffffff813e55c5>] xenbus_register_driver_common+0x15/0x20
 [<ffffffff813e76b3>] xenbus_register_frontend+0x23/0x40
 [<ffffffffa0015000>] ? 0xffffffffa0014fff
 [<ffffffffa001502b>] xenkbd_init+0x2b/0x1000 [xen_kbdfront]
 [<ffffffff81002049>] do_one_initcall+0x49/0x170

.. snip..

which is hardly nice. This patch fixes this by having each
PV driver check for:
 - if running in PV, then it is fine to execute (as that is their
   native environment).
 - if running in HVM, check if user wanted 'xen_emul_unplug=never',
   in which case bail out and don't load any PV drivers.
 - if running in HVM, and if PCI device 5853:0001 (xen_platform_pci)
   does not exist, then bail out and not load PV drivers.
 - (v2) if running in HVM, and if the user wanted 'xen_emul_unplug=ide-disks',
   then bail out for all PV devices _except_ the block one.
   Ditto for the network one ('nics').
 - (v2) if running in HVM, and if the user wanted 'xen_emul_unplug=unnecessary'
   then load block PV driver, and also setup the legacy IDE paths.
   In (v3) make it actually load PV drivers.

Reported-by: Sander Eikelenboom <linux@eikelenboom.it
Reported-by: Anthony PERARD <anthony.perard@citrix.com>
Reported-and-Tested-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[v2: Add extra logic to handle the myrid ways 'xen_emul_unplug'
can be used per Ian and Stefano suggestion]
[v3: Make the unnecessary case work properly]
[v4: s/disks/ide-disks/ spotted by Fabio]
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com> [for PCI parts]
CC: stable@vger.kernel.org