Re: [PATCH 1/4] Fix tboot enabled macro

[Jan Kiszka <jan.kiszka@siemens.com>]

Wang, Shane wrote:
> Jan Kiszka wrote:
>> Wang, Shane wrote:
>>> Avi Kivity wrote:
>>>> On 05/26/2010 10:25 AM, Jan Kiszka wrote:
>>>>> This is for CONFIG_INTEL_TXT enabled? Good point but needs to be
>>>>> solved differently. tboot, the variable that is checked by the
>>>>> original header, is not exported to modules. I wonder how this
>>>>> worked out for you... 
>>>>>
>>>>> Solution should be: hack tboot_enabled to kvm_tboot_enabled and
>>>>> unconditionally define that to 0 for older kernels. If tboot is
>>>>> actually enabled in hardware, KVM may not load but I'm unsure if
>>>>> it's OK to assume tboot == 1 for that case or if that will cause
>>>>> breakages if it's off instead - CC'ing the KVM patch author.
>>>>>
>>>> Worst case it doesn't load.  I don't think it's a problem since
>>>> enabling tboot will be rare for older kernels.
>>> tboot is not 0 if tboot module is run before kernel.
>>> If "tboot is enabled in hardware" (I assume you mean if Intel TXT is
>>> enabled in hardware) but tboot module is not run or old kernels
>>> don't support tboot module, 
>>> we still have outside_smx bit in feature msr. Why might KVM not load?
>> If we have to hard-wire tboot_enabled in kvm-kmod to 0, KVM may not
>> test all required bits and erroneously assume VTX would be disabled.
>>
>> So I wondered what would happen if we hard-wired it to 1, pretending
>> that the tboot modules is loaded. Would we gain something without
>> loosing on some other end? If not, I would simply leave things as they
>> are now (i.e. always assuming tboot absence).
>>
>> Thanks,
>> Jan
> 
> Why is VTX assumed to be disabled?

If TXT is on and VT is locked but KVM sees tboot_enabled == 0, it won't
check for FEATURE_CONTROL_VMXON_ENABLED_INSIDE_SMX during setup and may
consider VT unavailable. Moreover, if VT is not locked in that case, KVM
will also not set FEATURE_CONTROL_VMXON_ENABLED_INSIDE_SMX during
hardware_enable, likely leaving VT off then, no?

So my question is: Would it cause any harm to assume TXT being always
on, even if it wasn't?

Jan

> tboot_enabled == 0 but (msr & FEATURE_CONTROL_VMXON_ENABLED_OUTSIDE_SMX) == 1 if you have VT enabled.
> If you have VT enabled, VMX outside SMX is 1 always.
> 
> Shane

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux