Re: Detecting whether dom0 is in a VM

[Yann Dirson <yann.dirson@xxxxxxxx>]

On 7/6/23 17:35, zithro wrote:
> On 06 Jul 2023 09:02, Jan Beulich wrote:
>> On 05.07.2023 18:20, zithro wrote:
>>> So I'm wondering, isn't that path enough for correct detection ?
>>> I mean, if "/sys/class/dmi/id/sys_vendor" reports Xen (or KVM, or any
>>> other known hypervisor), it's nested, otherwise it's on hardware ?
>>>
>>> Is that really mandatory to use CPUID leaves ?
>>
>> Let me ask the other way around: In user mode code under a non-nested
>> vs nested Xen, what would you be able to derive from CPUID? The
>> "hypervisor" bit is going to be set in both cases. (All assuming you
>> run on new enough hardware+Xen such that CPUID would be intercepted
>> even for PV.)
> 
> I'm a bit clueless about CPUID stuff, but if I understand correctly, 
> you're essentially saying that using CPUID may not be the perfect way ?
> Also, I don't get why the cpuid command returns two different values, 
> depending on the -k switch :
> # cpuid -l 0x40000000
> hypervisor_id (0x40000000) = "\0\0\0\0\0\0\0\0\0\0\0\0"
> # cpuid -k -l 0x40000000
> hypervisor_id (0x40000000) = "XenVMMXenVMM"
> 
>> Yet relying on DMI is fragile, too: Along the lines of
>> https://lists.xen.org/archives/html/xen-devel/2022-01/msg00604.html
>> basically any value in there could be "inherited" from the host (i.e.
>> from the layer below, to be precise).
> 
> So using "/sys/class/dmi/id/sys_vendor", or simply doing "dmesg | grep 
> DMI:" is also not perfect, as values can be inherited/spoofed by 
> underneath hypervisor ?
> 
>> The only way to be reasonably
>> certain is to ask Xen about its view. The raw or host featuresets
>> should give you this information, in the "mirror" of said respective
>> CPUID leave's "hypervisor" bit. 
> 
> As said above, I'm clueless, can you expand please ?
> 
>> But of course that still won't tell
>> you which _kind_ of hypervisor is the immediate next one underneath
>> Xen.
>>
>> This then further raises the question of what use it is to know the
>> kind of the next level hypervisor, when multiple may be stacked on
>> top of one another ...
> 
> We need an answer from systemd guys, but the commiter expanded on the 
> reasons why the change was made [1] : "the detect_vm_cpuid check was 
> returning a VIRTUALIZATION_NONE result on non-nested dom0 (checking the 
> log from back then I was getting No virtualization found in CPUID), but 
> would report other CPUID-detectable hypervisors when dom0 was nested, so 
> we still wanted to check it for this case".
> 
> Systemd is using this information to not start some services when 
> nested, like SMART/smartmontools (which have 
> "ConditionVirtualization=no" in their systemd unit files).
> As the check now fails on baremetal dom0s, the service is not starting.

Is this really wise?  With any passed-through device the user likely 
wants such a service to start.  Are they not misled with this effort?

> 
> [1] https://github.com/systemd/systemd/issues/28113#issuecomment-1621559642
> 

-- 
Yann Dirson | Vates Platform Developer
XCP-ng & Xen Orchestra - Vates solutions
w: vates.tech | xcp-ng.org | xen-orchestra.com


Yann Dirson | Vates Platform Developer

XCP-ng & Xen Orchestra - Vates solutions
web: https://vates.tech