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: >>> 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. 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
