X86: Add a check to catch Xen emulation of Hyper-V

Xen emulates Hyper-V to host enlightened Windows. Looks like this
emulation may be turned on by default even for Linux guests. Check and
fail Hyper-V detection if we are on Xen.

[ hpa: the problem here is that Xen doesn't emulate Hyper-V well
  enough, and if the Xen support isn't compiled in, we end up stubling
  over the Hyper-V emulation and try to activate it -- and it fails. ]

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Link: http://lkml.kernel.org/r/1359940959-32168-2-git-send-email-kys@microsoft.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 646d192b18a2..4dab317f79b9 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -30,6 +30,13 @@ static bool __init ms_hyperv_platform(void)
         if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
                 return false;
 
+        /*
+         * Xen emulates Hyper-V to support enlightened Windows.
+         * Check to see first if we are on a Xen Hypervisor.
+         */
+        if (xen_cpuid_base())
+                return false;
+
         cpuid(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS,
               &eax, &hyp_signature[0], &hyp_signature[1], &hyp_signature[2]);