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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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