Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
IronPort-SDR: JT1EjmXoZX7i3KQr4AMz3khO5nf8k48sKOuJCqsiVkZP6pjmO7D1PRaKi8+13Hv/xiKi6YE+96
 WIxQ5XL3TClA==
IronPort-SDR: z63+64e3RqoyP/cB1ZU63XoHj3mn7H9ffeDUndTqcCXJkjxXi42qTdysLI/kBuObKpUHpHXYU/
 d8F/I/Ytuo3g==
Date:   Fri, 22 May 2020 15:20:55 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>
Subject: Re: [RFC][PATCH 0/4] x86/entry: disallow #DB more
Message-ID: <20200522222055.GE25128@linux.intel.com>
References: <20200522204738.645043059@infradead.org>
 <CALCETrV7GYg5V5dgM9BToc6RAqpcjRdoZoeXbnrTKTqjBfft6g@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CALCETrV7GYg5V5dgM9BToc6RAqpcjRdoZoeXbnrTKTqjBfft6g@mail.gmail.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.