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.
