Re: [RFC][PATCH 0/4] x86/entry: disallow #DB more

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, May 22, 2020 at 03:13:57PM -0700, Andy Lutomirski wrote:
> On Fri, May 22, 2020 at 1:49 PM Peter Zijlstra <peterz@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.