On Mon, Mar 28, 2022, Oliver Upton wrote: > While I was looking at #UD under nested for this issue, I noticed: > > Isn't there a subtle inversion on #UD intercepts for nVMX? L1 gets first dibs > on #UD, even though it is possible that L0 was emulating an instruction not > present in hardware (like RDPID). If L1 passed through RDPID the #UD > should not be reflected to L1. Yes, it's a known bug. > I believe this would require that we make the emulator aware of nVMX which > sounds like a science project on its own. I don't think it would require any new awareness in the emulator proper, KVM would "just" need to ensure it properly morphs the resulting reflected #UD to a nested VM-Exit if the emulator doesn't "handle" the #UD. In theory, that should Just Work... > Do we write this off as another erratum of KVM's (virtual) hardware on VMX? :) I don't think we write it off entirely, but it's definitely on the backburner because there are so precious few cases where KVM emulates on #UD. And for good reason, e.g. the RDPID case takes an instruction that exists purely to optimize certain flows and turns them into dreadfully sloooow paths.
