On Fri, 5 Aug 2022 19:00:12 -0700, Nadav Amit <nadav.amit@xxxxxxxxx> wrote: > On Aug 5, 2022, at 12:59 PM, Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Fri, Aug 05, 2022, Michal Luczaj wrote: > >> On Fri, 5 Aug 2022 13:42:40 +0200, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > >>> 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. > > If history is discussed, the test was created long before FEP. Without FEP, > the way to force the emulator to emulate an instruction was to set the > instruction in memory that is not mapped to the guest. But, as Sean stated, > this test always triggers #UD, so it was not necessary. > > The purpose of this test was to check a KVM fix for a bug that was found > during fuzzing: > > https://lore.kernel.org/all/5475DC42.6000201@xxxxxxxxxx/T/#m3a0da02d7c750c28816b08c43cf2ca03252b8bad OK, I think I finally get it. Thank you, guys, for all the details. Michal
