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? We have cpu_dr7, and some minor changes would make this work. Maybe replace all the direct cpu_dr7 access with helpers like dr7_set_bits() and dr7_clear_bits()? Also, I like raving at DR7 :)
