From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 01162C433DF for ; Sat, 23 May 2020 21:32:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C3D16207FB for ; Sat, 23 May 2020 21:32:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="Q7Sxl+I4" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388056AbgEWVco (ORCPT ); Sat, 23 May 2020 17:32:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60676 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387847AbgEWVcn (ORCPT ); Sat, 23 May 2020 17:32:43 -0400 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:e::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 69988C061A0E for ; Sat, 23 May 2020 14:32:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=euhs23l2LckQQZqClPP1ffQdVbjSsT+bd+0gLo+sW/g=; b=Q7Sxl+I4qXTrbIz8BvGb4iRFil QZQoRnqHO1Mz6Wkp21SyoFnG0Ji8aNikGKJt35awZetB2H3MMne3cBHkLncfp/dN+WYEPVzOwpYPE lBq5N0SnpFSutrz90UOxS65ArL74R5W5tL0UMYm/RZp7Fm1dBfXiE9qce5L2gl7VPcdZDFTrrEYk5 ZNi71itYo+xtF/ZJoWmnCBxIlvvahO8GJ8g4cYuxYPbMNtSZ3ug2VNDyodh99QI6QpjhuEv1i/guV t+NqbrNgCBLTILBO46cnwzZkwj70Ae3MGTNShVqbA4SLf/UqBn14BeZN+9VXe7SdM0mnICTjhEzsJ TYdZxkag==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jcbkz-0003mK-N4; Sat, 23 May 2020 21:32:37 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id 184D69834A2; Sat, 23 May 2020 23:32:35 +0200 (CEST) Date: Sat, 23 May 2020 23:32:35 +0200 From: Peter Zijlstra To: Andy Lutomirski Cc: Thomas Gleixner , LKML , X86 ML Subject: Re: [RFC][PATCH 0/4] x86/entry: disallow #DB more Message-ID: <20200523213235.GB4496@worktop.programming.kicks-ass.net> References: <20200522204738.645043059@infradead.org> <20200523125940.GA2483@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200523125940.GA2483@worktop.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 23, 2020 at 02:59:40PM +0200, Peter Zijlstra wrote: > On Fri, May 22, 2020 at 03:13:57PM -0700, Andy Lutomirski wrote: > > This is great, except that the unconditional DR7 write is going to > > seriously hurt perf performance. Fortunately, no one cares about > > perf, right? :) > > Good point, so the trivial optimization is below. I couldn't find > instruction latency numbers for DRn load/stores anywhere. I'm hoping > loads are cheap. + u64 empty = 0, read = 0, write = 0; + unsigned long dr7; + + for (i=0; i<100; i++) { + u64 s; + + s = rdtsc(); + barrier_nospec(); + barrier_nospec(); + empty += rdtsc() - s; + + s = rdtsc(); + barrier_nospec(); + dr7 = native_get_debugreg(7); + barrier_nospec(); + read += rdtsc() - s; + + s = rdtsc(); + barrier_nospec(); + native_set_debugreg(7, 0); + barrier_nospec(); + write += rdtsc() - s; + } + + printk("XXX: %ld %ld %ld\n", empty, read, write); [ 1.628125] XXX: 2800 2404 19600 IOW, reading DR7 is basically free, and certainly cheaper than looking at cpu_dr7 which would probably be an insta cache miss. Which seems to suggest KVM can go pound sand. Maybe they can fix themselves with some paravirt love.