[Lguest] lguest: unhandled trap 13

[Avi Kivity]

On 02/11/2010 03:30 AM, Rusty Russell wrote:
> On Thu, 11 Feb 2010 07:30:06 am Avi Kivity wrote:
>    
>> On 02/10/2010 04:33 AM, Rusty Russell wrote:
>>      
>>> Note that lguest doesn't run under kvm (I should really file a kvm bug)
>>>        
>> Yes please.
>>
>> How does it fail?
>>      
> I was hoping to get cycles to look at it myself.
>
> Host (ie Linux inside kvm) doesn't see #GP when guest (lguest, Linux ring 1)
> invokes KVM_HYPERCALL.  Guest continues execution after hypercall.
>
> One solution would be to return to lguest-specific hypercall mechanism, but
> the faux commonality is cute.
>
>    

The VMCALL documentation says:

> IF not in VMX operation
>     THEN #UD;
> ELSIF in VMX non-root operation
>     THEN VM exit;
> ELSIF (RFLAGS.VM = 1) OR (IA32_EFER.LMA = 1 and CS.L = 0)
>     THEN #UD;
> ELSIF CPL > 0
>     THEN #GP(0);

So there's no reason to expect a #GP if running in a guest.

Best for performance would be to use SYSCALL/SYSENTER, but that means 
modifying the ordinary syscall fastpath, which isn't nice.  You could 
switch the syscall entry point msr, but that's slow.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.