Re: [PATCH 2/2] KVM: selftests: x86: Test illegal LEA handling

[Sean Christopherson <seanjc@google.com>]

On Fri, Jul 29, 2022, Michal Luczaj wrote:
> + * To trigger the emulator and feed it with LEA, we VM-exit on IO (with a
> + * single OUTS), then race decoder's instruction fetch - hoping to replace the
> + * initial IO op with an illegal LEA.

Rather than play games with memory, can't we just require and use force_emulation_prefix
to force KVM to emulate a bogus LEA encoding?  emulator.c in KVM-unit-tests already has
most of what you need, e.g. I believe it's just a matter of implementing
test_illegal_lea().  That test already has test_smsw_reg(), which is darn near the
same thing, it just expects a different result (success instead of #UD).

diff --git a/x86/emulator.c b/x86/emulator.c
index cd78e3cb..dd50578d 100644
--- a/x86/emulator.c
+++ b/x86/emulator.c
@@ -1193,6 +1193,7 @@ int main(void)
                test_smsw_reg(mem);
                test_nop(mem);
                test_mov_dr(mem);
+               test_illegal_lea();
        } else {
                report_skip("skipping register-only tests, "
                            "use kvm.force_emulation_prefix=1 to enable");