Re: [Qemu-devel] Re: [PATCH 1/2] allow hypervisor CPUID bit to be overriden

[Jamie Lokier]

Avi Kivity wrote:
> On 06/23/2009 02:31 PM, Paul Brook wrote:
> >On Tuesday 23 June 2009, Avi Kivity wrote:
> >   
> >>On 06/23/2009 12:47 AM, Andre Przywara wrote:
> >>     
> >>>KVM defaults to the hypervisor CPUID bit to be set, whereas pure QEMU
> >>>clears it. On some occasions one want to set or clear it the other way
> >>>round (for instance to get HyperV running inside a guest).
> >>>Allow the default to be overridden on the command line and fix some
> >>>whitespace damage on the way.
> >>>       
> >>It makes sense for qemu to set the hypervisor bit unconditionally.  A
> >>guest running under qemu is not bare metal.
> >>     
> >
> >I see no reason why a guest has to be told that it's running inside a VM.
> >In principle an appropriately configured qemu should be indistinguishable 
> >from
> >real hardware. In practice it's technically infeasible to cover absolutely
> >everything, but if we set this bit we're not even trying.
> >
> >I have no objection to the bit being set by default for the QEMU CPU types.
> >   
> 
> I agree it's pointless, but it is a Microsoft requirement for passing 
> their SVVP tests.  Enabling it by default makes life a little easier for 
> users who wish to validate their hypervisor and has no drawbacks.

Hold on.

Do the SVVP tests fail on a real (non-virtal) machine then?

Or is QEMU's machine emulation insufficiently accurate?

I see a drawback in setting the bit by default.

Something I expect from an emulator is that it behaves like a real
machine to the extent possible.  In particular, guest code which
attempts to check if it's running on a real machine should get the
answer "yes".  Unfriendly guest code which pops up a message like
"Sorry I refuse to work for you after 100 hours/ because you are
attempting to run me in a virtual machine, and don't even think of
trying to hide this from me now you know I look for it" should never
do so.

-- Jamie