Jan Kiszka <jan.kiszka@xxxxxxxxxxx> writes:
> On 2015-02-24 17:30, Radim Krčmář wrote:
>> 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
>
> Reminds me of a patch called "KVM: nVMX: Disable unrestricted mode if
> ept=0" by Bandan. I thought that would have caught it - apparently not.
Yeah... Unrestricted guest could be disabled even if ept=0,
and I incorrectly didn't take that into account.
>> 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.
>
> Agreed.
>
> Jan
>
>>
>> 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@xxxxxxxxxx>
We should Cc stable on this patch.
Bandan
>> ---
>> 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,
>>
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