On Mon, May 23, 2022, Chenyi Qiang wrote:
>
>
> On 5/19/2022 3:20 AM, Sean Christopherson wrote:
> > On Thu, Apr 21, 2022, Chenyi Qiang wrote:
> > > +#ifndef __x86_64__
> > > +# error This test is 64-bit only
> >
> > No need, all of KVM selftests are 64-bit only.
> >
> > > +#endif
> > > +
> > > +#define VCPU_ID 0
> > > +#define ARBITRARY_IO_PORT 0x2000
> > > +
> > > +/* The virtual machine object. */
> > > +static struct kvm_vm *vm;
> > > +
> > > +static void l2_guest_code(void)
> > > +{
> > > + /*
> > > + * Generate an exit to L0 userspace, i.e. main(), via I/O to an
> > > + * arbitrary port.
> > > + */
> >
> > I think we can test a "real" triple fault without too much effort by abusing
> > vcpu->run->request_interrupt_window. E.g. map the run struct into L2, clear
> > PIN_BASED_EXT_INTR_MASK in vmcs12, and then in l2_guest_code() do:
> >
> > asm volatile("cli");
> >
> > run->request_interrupt_window = true;
> >
>
> Maybe, A GUEST_SYNC to main() to change the request_interrupt_window also
> works.
Hmm, yes, that should work too. Feel free to punt on writing this sub-test. As
mentioned below, KVM should treat a pending triple fault as a pending "exception",
i.e. userspace shouldn't see KVM_EXIT_IRQ_WINDOW_OPEN with a pending triple fault,
and so the test should actually assert that the triple fault occurs in L2. But I
can roll that test into the KVM fix if you'd like.
> > asm volatile("sti; ud2");
> >
>
> How does the "real" triple fault occur here? Through UD2?
Yeah. IIRC, by default L2 doesn't have a valid IDT, so any fault will escalate to
a triple fault.
> > asm volatile("inb %%dx, %%al"
> > : : [port] "d" (ARBITRARY_IO_PORT) : "rax");
> >
> > The STI shadow will block the IRQ-window VM-Exit until after the ud2, i.e. until
> > after the triple fault is pending.
> >
> > And then main() can
> >
> > 1. verify it got KVM_EXIT_IRQ_WINDOW_OPEN with a pending triple fault
> > 2. clear the triple fault and re-enter L1+l2
> > 3. continue with the existing code, e.g. verify it got KVM_EXIT_IO, stuff a
> > pending triple fault, etc...
> >
> > That said, typing that out makes me think we should technically handle/prevent this
> > in KVM since triple fault / shutdown is kinda sorta just a special exception.
