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Subject[tip:x86/hyperv] X86: Add a check to catch Xen emulation of Hyper-V
Commit-ID:  db34bbb767bdfa1ebed7214b876fe01c5b7ee457
Author: K. Y. Srinivasan <>
AuthorDate: Sun, 3 Feb 2013 17:22:38 -0800
Committer: H. Peter Anvin <>
CommitDate: Tue, 12 Feb 2013 16:27:03 -0800

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 <>
Signed-off-by: H. Peter Anvin <>
arch/x86/kernel/cpu/mshyperv.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 646d192..4dab317 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;
&eax, &hyp_signature[0], &hyp_signature[1], &hyp_signature[2]);

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