Re: [Xen-devel] [PATCH] tools: fix xen-detect to correctly identify domU type

[Juergen Gross <jgross@xxxxxxxx>]

On 23/03/16 10:10, Jan Beulich wrote:
>>>> On 23.03.16 at 08:50, <JGross@xxxxxxxx> wrote:
>> xen-detect always thinks a domU is running as HVM guest as the cpuid
>> instruction used to identify a Xen guest will always work.
>>
>> In order to identify a pv guest first try the pv special case of
>> cpuid (prefixed with an ud2a instruction and "xen" in ASCII). This
>> will fail on HVM and thus can be used to distinguish the guest types.
> 
> I don't think that's something to be relied upon, or even true
> universally (see hvm_ud_intercept()). With CPUID faulting in mind
> I can't see a purely CPUID based mechanism that would reliably
> allow to tell one from the other as well as from PVH/PVHv2. There
> is one feature flag specifically getting set for PV domains only
> (XEN_CPUID_FEAT1_MMU_PT_UPDATE_PRESERVE_AD), but that
> won't be available on older hypervisors afaict.

So this would mean we have the following options:

a) nuke xen-detect completely, as it is unreliable
b) do the modification I've suggested, being better than the current
   version
c) modify it to ask the OS (is there an interface available we can
   use?)

Thoughts?


Juergen


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