Re: [PATCH 4/5] KVM: selftests: Explicitly verify KVM doesn't patch hypercall if quirk==off

Sean Christopherson
2022-09-20T18:46:38Z

On Mon, Sep 19, 2022, Oliver Upton wrote:
> On Thu, Sep 08, 2022 at 11:31:33PM +0000, Sean Christopherson wrote:
> > @@ -75,12 +76,28 @@ static void guest_main(void)
> > }
> >
> > /*
> > - * The hypercall didn't #UD (guest_ud_handler() signals "done" if a #UD
> > - * occurs). Verify that a #UD is NOT expected and that KVM patched in
> > - * the native hypercall.
> > + * If the quirk is disabled, verify that guest_ud_handler() "returned"
> > + * -EFAULT and that KVM did NOT patch the hypercall. If the quirk is
> > + * enabled, verify that the hypercall succeeded and that KVM patched in
> > + * the "right" hypercall.
> > */
> > - GUEST_ASSERT(!ud_expected);
> > - GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn, HYPERCALL_INSN_SIZE));
> > + if (ud_expected) {
> > + GUEST_ASSERT(ret == (uint64_t)-EFAULT);
> > +
> > + /*
> > + * Divergence should occur only on the last byte, as the VMCALL
> > + * (0F 01 C1) and VMMCALL (0F 01 D9) share the first two bytes.
> > + */
> > + GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn,
> > + HYPERCALL_INSN_SIZE - 1));
> > + GUEST_ASSERT(memcmp(native_hypercall_insn, hypercall_insn,
> > + HYPERCALL_INSN_SIZE));
>
> Should we just keep the assertions consistent for both cases (patched
> and unpatched)?

Not sure I follow what you're suggesting. By "consistent" do you mean doing
something like snapshotting hypercall_insn and verifying that it's not changed?