Re: [Qemu-devel] [visorchipset] invalid opcode: 0000 [#1] PREEMPT SMP

[Paolo Bonzini]

Il 11/04/2014 19:40, H. Peter Anvin ha scritto:
> On 04/11/2014 10:35 AM, Jet Chen wrote:
> > As Peter said, QEMU probably should *not* set the hypervisor bit. But based on 
> > my testing, I think KVM works properly in this case.
>
> Either way, unless there is a CPUID interface exposed in CPUID levels
> 0x40000000+, then relying on the hypervisor bit to do VMCALL is wrong in
> the extreme.

Sorry for the delay guys, I was on vacation.

Lack of a CPUID interface at 0x40000000 is indeed *the* good reason why 
QEMU should not set the hypervisor bit.  Of course that there is no 
guarantee that QEMU will never expose a 0x40000000 interface, and at 
that point the hypervisor bit may reappear in QEMU's JIT mode.

As to sending #UD to the guest at CPL>0, that is a choice of the 
hypervisor.  Hyper-V (and KVM in Hyper-V emulation mode) does that, and 
does the same in real mode too.  KVM instead sets EAX to -KVM_EPERM, and 
accepts hypercalls in real mode (where CPL=0).  Terminating the guest is 
surely the wrong thing to do at CPL>0.

Thanks,

Paolo