[PATCHv2] KVM: x86 emulator: fix xchg instruction emulation

[Wei Yongjun <yjwei@cn.fujitsu.com>]

If the destination is a memory operand and the memory cannot
map to a valid page, the xchg instruction emulation and locked
instruction will not work on io regions and stuck in endless
loop. We should emulate exchange as write to fix it.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Gleb Natapov <gleb@redhat.com>
---
 arch/x86/kvm/x86.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 86c8102..84bfb51 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3562,6 +3562,10 @@ static int emulator_cmpxchg_emulated(unsigned long addr,
 		goto emul_write;
 
 	page = gfn_to_page(vcpu->kvm, gpa >> PAGE_SHIFT);
+	if (is_error_page(page)) {
+		kvm_release_page_clean(page);
+		goto emul_write;
+	}
 
 	kaddr = kmap_atomic(page, KM_USER0);
 	kaddr += offset_in_page(gpa);
-- 
1.7.0.4