Re: KVM exit on UD interception

["Nakajima, Jun" <jun.nakajima@intel.com>]

On Mon, May 5, 2014 at 8:56 AM, Alexandru Duţu <alex.dutu@gmail.com> wrote:
> Dear all,
>
> It seems that currently, on UD interception KVM does not exit
> completely. Virtualized execution finishes, KVM executes
> ud_intercept() after which it enters virtualized execution again.

Maybe you might want to take a look at the VMX side (to port it to
SVM). The MOVBE emulation, for example, should be helpful.

>
> I am working on accelerating with virtualized execution a simulator
> that emulates system calls. Essentially doing virtualized execution
> without a OS kernel. In order to make this work, I had to modify my
> the KVM kernel module such that ud_intercept() return 0 and not 1
> which break KVM __vcpu_run loop. This is necessary as I need to trap
> syscall instructions, exit virtualized execution with UD exception,
> emulate the system call in the simulator and after the system call is
> done enter back in virtualized mode and start execution with the help
> of KVM.
>
> So by modifying ud_intercept() to return 0, I got all this to work. Is
> it possible to achieve the same effect (exit on undefined opcode)
> without modifying ud_intercept()?
>
> It seems that re-entering virtualized execution on UD interception
> gives the user the flexibility of running binaries with newer
> instructions on older hardware, if kvm is able to emulate the newer
> instructions. I do not fully understand the details of this scenario,
> is there such a scenario or is it likely that ud_interception() will
> change?
>
> Thank you in advance!
>
> Best regards,
> Alex
> --

-- 
Jun
Intel Open Source Technology Center