> On May 22, 2020, at 3:20 PM, Sean Christopherson > <sean.j.christopher...@intel.com> wrote: > > On Fri, May 22, 2020 at 03:13:57PM -0700, Andy Lutomirski wrote: >>> On Fri, May 22, 2020 at 1:49 PM Peter Zijlstra <pet...@infradead.org> wrote: >>> >>> Hai, this kills #DB during NMI/#MC and with that allows removing all the >>> nasty >>> IST rewrite crud. >>> >> >> This is great, except that the unconditional DR7 write is going to >> seriously hurt perf performance. Fortunately, no one cares about >> perf, right? :) Even just reading first won't help enough because DR7 >> reads are likely to be VM exits. Can we have a percpu dr7 shadow >> (with careful ordering) or even just a percpu count of dr7 users so we >> can skip this if there are no breakpoints? > > Hmm, I believe hw_breakpoint_active() is what you're looking for, KVM uses > it to avoid unnecessary restoration of host DR7 after VM-Exit. > > Amusingly, checking that in the NMI handler could give a false positive if > an NMI occurs in guest as DR7 is cleared on exit and KVM invokes the NMI > handler prior to restoring host DR7. I doubt that's common enough to care > about though.
False positives are unavoidable: there’s no way we can set a percpu variable and set DR7 without risking an NMI in between.
