Re: [Xen-devel] [PATCH 2/2] xen-detect: check for XEN_PV before XEN_HVM

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/12/14 14:37, John Haxby wrote:
> At some stage, the cpuid instruction used to detect a xen hvm domain
> also started working in a pv domain so pv domains were being identified
> as hvm (dom0 excepted).  Change the order so that pv is tested for
> first.
>
> Signed-off-by: John Haxby <john.haxby@xxxxxxxxxx>

This will have happened as a side effect of Intels CPUID-faulting
ability present in IvyBridge servers and later, which permits Xen the
ability to intercept regular cpuid instructions.

On the other hand, the forced emulation prefix is now valid in HVM
guests in debug Xens with the "hvm_fep" command line option.  In that
case, the HVM domain would be erroneously identified as PV.

Perhaps it is worth having an explicit guest type available in the cpuid
leaves themselves, so guest userspace need not guess at all?

~Andrew


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