On Tue, Mar 16, 2021, Maxim Levitsky wrote: > On Tue, 2021-03-16 at 16:31 +0100, Jan Kiszka wrote: > > Back then, when I was hacking on the gdb-stub and KVM support, the > > monitor trap flag was not yet broadly available, but the idea to once > > use it was already there. Now it can be considered broadly available, > > but it would still require some changes to get it in. > > > > Unfortunately, we don't have such thing with SVM, even recent versions, > > right? So, a proper way of avoiding diverting event injections while we > > are having the guest in an "incorrect" state should definitely be the goal. > Yes, I am not aware of anything like monitor trap on SVM. > > > > > Given that KVM knows whether TF originates solely from guest debugging > > or was (also) injected by the guest, we should be able to identify the > > cases where your approach is best to apply. And that without any extra > > control knob that everyone will only forget to set. > Well I think that the downside of this patch is that the user might actually > want to single step into an interrupt handler, and this patch makes it a bit > more complicated, and changes the default behavior. Yes. And, as is, this also blocks NMIs and SMIs. I suspect it also doesn't prevent weirdness if the guest is running in L2, since IRQs for L1 will cause exits from L2 during nested_ops->check_events(). > I have no objections though to use this patch as is, or at least make this > the new default with a new flag to override this. That's less bad, but IMO still violates the principle of least surprise, e.g. someone that is single-stepping a guest and is expecting an IRQ to fire will be all kinds of confused if they see all the proper IRR, ISR, EFLAGS.IF, etc... settings, but no interrupt. > Sean Christopherson, what do you think? Rather than block all events in KVM, what about having QEMU "pause" the timer? E.g. save MSR_TSC_DEADLINE and APIC_TMICT (or inspect the guest to find out which flavor it's using), clear them to zero, then restore both when single-stepping is disabled. I think that will work?
