Re: [kvm-unit-tests PATCH 2/4] x86: emulator.c cleanup: Use ASM_TRY for the UD_VECTOR cases

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 05, 2022, Michal Luczaj wrote:
> On Fri, 5 Aug 2022 13:42:40 +0200, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > On 8/3/22 20:21, Sean Christopherson wrote:
> > >> I've noticed test_illegal_movbe() does not execute with KVM_FEP.
> > >> Just making sure: is it intentional?
> > > It's intentional.  FEP isn't needed because KVM emulates MOVBE on #UD when it's
> > > not supported by the host, e.g. to allow migrating to an older host.
> > > 
> > > 	GP(EmulateOnUD | ModRM, &three_byte_0f_38_f0),
> > > 	GP(EmulateOnUD | ModRM, &three_byte_0f_38_f1),
> > > 
> > 
> > *puts historian hat on*
> > 
> > The original reason was to test Linux using MOVBE even on non-Atom 
> > machines, when MOVBE was only on Atoms. :)
> 
> So the emulator's logic for MOVBE is meant to be tested only when the
> guest supports MOVBE while the host does not?

Ah, I see what you're asking.  No, it's perfectly legal to test MOVBE emulation
on hosts that support MOVBE, i.e. using FEP is allowed.  But because KVM emulates
MOVBE on #UD and the KUT testcase is guaranteed to generate a #UD (barring a
hardware bug), there's no need to use FEP.  And not using FEP is advantageous
because it avoids depending on an opt-in non-production module param.