On 16.03.21 17:50, Sean Christopherson wrote: > 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. >From my practical experience with debugging guests via single step, seeing an interrupt in that case is everything but handy and generally also not expected (though logical, I agree). IOW: When there is a knob for it, it will remain off in 99% of the time. But I see the point of having some control, in an ideal world also an indication that there are pending events, permitting the user to decide what to do. But I suspect the gdb frontend and protocol does not easily permit that. > >> 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? > No one can stop the clock, and timers are only one source of interrupts. Plus they do not all come from QEMU, some also from KVM or in-kernel sources directly. Would quickly become a mess. Jan -- Siemens AG, T RDA IOT Corporate Competence Center Embedded Linux
