[Qemu-devel] Re: SVM emulation: EVENTINJ marked valid when a pagefault happens while issuing a software interrupt

[Roedel, Joerg]

On Thu, May 27, 2010 at 06:20:00PM -0400, Jan Kiszka wrote:
> Erik van der Kouwe wrote:
> > Problem is: I'm compiling in Linux and testing in MINIX. Testing on the
> > real hardware would require a reboot everytime. Moreover, it might screw
> > up my system if I make bad mistakes (the MINIX filesystem is easily
> > corrupted).
> 
> Use Linux+KVM as host OS, it can also run VMMs as guests (aka nested
> SVM). And you could even debug those guests just like when you would run
> QEMU in emulation mode. In contrast to SVM emulation, nesting is fairly
> stable AFAIK. And it is faster.

At least it is more stable than any other nested-svm implementation I
know of ;-)
There are issues with kvmclock when you run kvm-on-kvm and you should
not expect windows-based hypervisors to run without problems. Beside
that, for running kvm-on-kvm and xen-on-kvm it is indeed fairly
stable :-)

> > Linux source tree (2.6.31-ubuntu), arch/x86/kvm/svm.c, end of function
> > nested_svm_vmrun. Here event_inj and event_inj_err are copied from a
> > different VMCB, effectively clearing the value set by the CPU. Maybe
> > this isn't were I should have been looking though?

The interesting part is in nested_svm_vmexit. There you have this piece
of code:

        /*
         * If we emulate a VMRUN/#VMEXIT in the same host #vmexit cycle we have
         * to make sure that we do not lose injected events. So check event_inj
         * here and copy it to exit_int_info if it is valid.
         * Exit_int_info and event_inj can't be both valid because the case
         * below only happens on a VMRUN instruction intercept which has
         * no valid exit_int_info set.
         */
        if (vmcb->control.event_inj & SVM_EVTINJ_VALID) {
                struct vmcb_control_area *nc = &nested_vmcb->control;

                nc->exit_int_info     = vmcb->control.event_inj;
                nc->exit_int_info_err = vmcb->control.event_inj_err;
        }

and a few lines later:

        nested_vmcb->control.event_inj         = 0;
        nested_vmcb->control.event_inj_err     = 0;

... which takes care of this situation. The vmcb.eventinf field is _defined_ to
be zero on a #vmexit.

        Joerg