[Fedora-xen] f10 x86_64 xen VM guests fail to boot on f8 host

[Avi Kivity]

Virtualization wrote:
> Hi list,
>
> >From the Intel® Virtualization Technology Specification
> for the IA-32 Intel® Architecture (2005):
>
> "2.9.2       Information for VM Exits Due to Vectored Events
> Event-specific information is provided for VM exits due to the following vectored events:
> exceptions (including those generated by the instructions INT3, INTO, BOUND, and UD2);
> external interrupts that occur while the “acknowledge interrupt on exit” VM-exit control is 1;
> and non-maskable interrupts (NMIs). This information is provided in the following fields:" ....
>
> The <0f> 0b in the "Code:" section are the UD2 instruction.
>
> Checking through the OpCode map for the Xeon processor, this is an
> invalid op code. In VT processors the software guide indicates that a
> program can communicate various events and state information to the
> underlying virtualization supervisor by executing a UD2 (and some others
> ops like it).
>
> I think that in a non-VT cpu it's actually a "real" invalid op code. The
> stuff (hardware) which flips over to the supervisor with all the needed
> info from the virtual machine isn't there.
>
> KVM uses this, from the patches I've seen Googling around for UD2 (if I
> understand correctly).
>
> So why a UD2 in the code? It's highly unlikely that it's just some
> random bytes that happen to be a UD2. Possibly the kernel thinks it's in
> fully virt mode at some point? The image notes do seem to indicate this.
>   

The guest kernel uses UD2 as part of the implementation of an 
assertion.  In this case, UD2 is used by the guest to communicate with 
itself.  Every BUG: report you see will have the code containing 0f 0b.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.