[Bug 106621] New: Failure to install Hyper-V role on nested KVM guest

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https://bugzilla.kernel.org/show_bug.cgi?id=106621

            Bug ID: 106621
           Summary: Failure to install Hyper-V role on nested KVM guest
           Product: Virtualization
           Version: unspecified
    Kernel Version: 4.3.0-rc7
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: kvm
          Assignee: virtualization_kvm@kernel-bugs.osdl.org
          Reporter: Rainmaker52@gmail.com
        Regression: No

When installing the Hyper-V role on a Windows guest (tried 2008 R2, 2012 R2 and
2016 TP3), the Windows installer refuses to install because of the error 

"Hyper-V cannot be installed because virtualization support is not enabled in
the BIOS."

This is because the MSR 0x3a is initialized to "0".

When VMX is activated on the guest CPU, the 0x3a register should return "5".

The following code patch (may be a bit of an overstatement) returns "5" if VMX
is set on the guest CPU, thereby reporting to the guest that visualization is
enabled in the BIOS. 

--- a/arch/x86/kvm/vmx.c    2015-10-25 02:39:47.000000000 +0100
+++ b/arch/x86/kvm/vmx.c    2015-10-26 13:35:51.894700786 +0100
@@ -2661,7 +2661,12 @@
     case MSR_IA32_FEATURE_CONTROL:
         if (!nested_vmx_allowed(vcpu))
             return 1;
-        msr_info->data = to_vmx(vcpu)->nested.msr_ia32_feature_control;
+        if (nested_vmx_allowed(vcpu)) {
+            //Set all 3 bits in 0x3a
+            msr_info->data = 5;
+        } else {
+            msr_info->data = to_vmx(vcpu)->nested.msr_ia32_feature_control;
+        }
         break;
     case MSR_IA32_VMX_BASIC ... MSR_IA32_VMX_VMFUNC:
         if (!nested_vmx_allowed(vcpu))


This, together with "-cpu host,-hypervisor,+vmx will allow Hyper-V to be
installed. It will however not allow to start these Virtual Machines.

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