Re: [syzbot] [kvm?] WARNING in vmx_handle_exit (2)

[James Houghton]

Here's what I think is going on (with the C repro anyway):

1. KVM_RUN a nested VM, and eventually we end up with
   nested_run_pending=1.
2. Exit KVM_RUN with EINTR (or any reason really, but I see EINTR in
   repro attempts).
3. KVM_SET_REGS to set rflags to 0x1ac585, which has X86_EFLAGS_VM,
   flipping it and setting vmx->emulation_required = true.
3. KVM_RUN again. vmx->emulation_required will stop KVM from clearing
   nested_run_pending, and then we hit the
   KVM_BUG_ON(nested_run_pending) in __vmx_handle_exit().

So I guess the KVM_BUG_ON() is a little bit too conservative, but this
is nonsensical VMM behavior. So I'm not really sure what the best
solution is. Sean, any thoughts?