From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stefan Bader <stefan.bader@canonical.com>
Subject: regression: nested: L1 3.15+ fails to load kvm-intel on L0 <3.15
Date: Wed, 18 Mar 2015 09:46:10 +0100
Message-ID: <55093B52.5090904@canonical.com>
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To: kvm@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>
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Someone reported[1] that some of their L1 guests fail to load the kvm-int=
el
module (without much details). Turns out that this was (at least) caused =
by

KVM: vmx: Allow the guest to run with dirty debug registers

as this adds VM_EXIT_SAVE_DEBUG_CONTROLS to the required MSR_IA32_VMX_EXI=
T_CTLS
bits. Not sure this should be fixed up in pre 3.15 kernels or the other w=
ay
round. Maybe naively asked but would it be sufficient to add this as requ=
ired to
older kernels vmcs setup (without the code to make any use of it)?

Regardless of that, I wonder whether the below (this version untested) so=
und
acceptable for upstream? At least it would make debugging much simpler. :=
)

--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2953,8 +2953,11 @@ static __init int adjust_vmx_controls(u32 ctl_min,=
 u32 ct
        ctl |=3D vmx_msr_low;  /* bit =3D=3D 1 in low word  =3D=3D> must =
be one  */

        /* Ensure minimum (required) set of control bits are supported. *=
/
-       if (ctl_min & ~ctl)
+       if (ctl_min & ~ctl) {
+               printk(KERN_ERR "vmx: msr(%08x) does not match requiremen=
ts. "
+                               "req=3D%08x cur=3D%08x\n", msr, ctl_min, =
ctl);
                return -EIO;
+       }

        *result =3D ctl;
        return 0;

Thanks,
-Stefan

[1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1431473


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