[PATCH] KVM: nVMX: mask unrestricted_guest if disabled on L0

["Radim Krčmář" <rkrcmar@redhat.com>]

2015-02-23 19:05+0100, Kashyap Chamarthy:
> Tested with the _correct_ Kernel[1] (that has Radim's patch) now --
> applied it on both L0 and L1.
> 
> Result: Same as before -- Booting L2 causes L1 to reboot. However, the
>         stack trace from `dmesg` on L0 is took slightly different path than
>         before -- it's using MSR handling:

Thanks, the problem was deeper ... L1 enabled unrestricted mode while L0
had it disabled.  L1 could then vmrun a L2 state that L0 would have to
emulate, but that doesn't work.  There are at least these solutions:

 1) don't expose unrestricted_guest when L0 doesn't have it
 2) fix unrestricted mode emulation code
 3) handle the failure a without killing L1

I'd do just (1) -- emulating unrestricted mode is a loss.

I have done initial testing and at least qemu-sanity-check works now:

---8<---
If EPT was enabled, unrestricted_guest was allowed in L1 regardless of
L0.  L1 triple faulted when running L2 guest that required emulation.

Another side effect was 'WARN_ON_ONCE(vmx->nested.nested_run_pending)'
in L0's dmesg:
  WARNING: CPU: 0 PID: 0 at arch/x86/kvm/vmx.c:9190 nested_vmx_vmexit+0x96e/0xb00 [kvm_intel] ()

Prevent this scenario by masking SECONDARY_EXEC_UNRESTRICTED_GUEST when
the host doesn't have it enabled.

Fixes: 78051e3b7e35 ("KVM: nVMX: Disable unrestricted mode if ept=0")
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
---
 arch/x86/kvm/vmx.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index f7b20b417a3a..dbabea21357b 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2476,8 +2476,7 @@ static void nested_vmx_setup_ctls_msrs(struct vcpu_vmx *vmx)
 	if (enable_ept) {
 		/* nested EPT: emulate EPT also to L1 */
 		vmx->nested.nested_vmx_secondary_ctls_high |=
-			SECONDARY_EXEC_ENABLE_EPT |
-			SECONDARY_EXEC_UNRESTRICTED_GUEST;
+			SECONDARY_EXEC_ENABLE_EPT;
 		vmx->nested.nested_vmx_ept_caps = VMX_EPT_PAGE_WALK_4_BIT |
 			 VMX_EPTP_WB_BIT | VMX_EPT_2MB_PAGE_BIT |
 			 VMX_EPT_INVEPT_BIT;
@@ -2491,6 +2490,10 @@ static void nested_vmx_setup_ctls_msrs(struct vcpu_vmx *vmx)
 	} else
 		vmx->nested.nested_vmx_ept_caps = 0;
 
+	if (enable_unrestricted_guest)
+		vmx->nested.nested_vmx_secondary_ctls_high |=
+			SECONDARY_EXEC_UNRESTRICTED_GUEST;
+
 	/* miscellaneous data */
 	rdmsr(MSR_IA32_VMX_MISC,
 		vmx->nested.nested_vmx_misc_low,