[PATCH] pvh: Fix regression caused by assumption that HVM paths MUST use io-backend device.

[Konrad Rzeszutek Wilk <konrad@kernel.org>]

The commit 09bb434748af9bfe3f7fca4b6eef721a7d5042a4
"Nested VMX: prohibit virtual vmentry/vmexit during IO emulation"
assumes that the HVM paths are only taken by HVM guests. With the PVH
enabled that is no longer the case - which means that we do not have
to have the IO-backend device (QEMU) enabled.

As such, that patch can crash the hypervisor:

Xen call trace:
    [<ffff82d0801ddd9a>] nvmx_switch_guest+0x4d/0x903
    [<ffff82d0801de95b>] vmx_asm_vmexit_handler+0x4b/0xc0

Pagetable walk from 000000000000001e:
  L4[0x000] = 0000000000000000 ffffffffffffffff

****************************************
Panic on CPU 7:
FATAL PAGE FAULT
[error_code=0000]
Faulting linear address: 000000000000001e
****************************************

as we do not have an io based backend.

CC: Yang Zhang <yang.z.zhang@Intel.com>
CC: Jun Nakajima <jun.nakajima@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 xen/arch/x86/hvm/vmx/vvmx.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vvmx.c b/xen/arch/x86/hvm/vmx/vvmx.c
index d2ba435..2f516c9 100644
--- a/xen/arch/x86/hvm/vmx/vvmx.c
+++ b/xen/arch/x86/hvm/vmx/vvmx.c
@@ -1400,7 +1400,7 @@ void nvmx_switch_guest(void)
      * no virtual vmswith is allowed. Or else, the following IO
      * emulation will handled in a wrong VCPU context.
      */
-    if ( get_ioreq(v)->state != STATE_IOREQ_NONE )
+    if ( get_ioreq(v) && get_ioreq(v)->state != STATE_IOREQ_NONE )
         return;
     /*
      * a softirq may interrupt us between a virtual vmentry is
-- 
1.7.7.6