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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 A2749C43387 for ; Mon, 7 Jan 2019 22:25:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 608292147C for ; Mon, 7 Jan 2019 22:25:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727157AbfAGWZa (ORCPT ); Mon, 7 Jan 2019 17:25:30 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43362 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726643AbfAGWZ3 (ORCPT ); Mon, 7 Jan 2019 17:25:29 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0939EC05D3FD; Mon, 7 Jan 2019 22:25:29 +0000 (UTC) Received: from redhat.com (ovpn-122-143.rdu2.redhat.com [10.10.122.143]) by smtp.corp.redhat.com (Postfix) with ESMTP id DFB94648DE; Mon, 7 Jan 2019 22:25:27 +0000 (UTC) Date: Mon, 7 Jan 2019 17:25:27 -0500 From: "Michael S. Tsirkin" To: Dan Williams Cc: Jason Wang , KVM list , virtualization@lists.linux-foundation.org, Netdev , Linux Kernel Mailing List , David Miller Subject: Re: [RFC PATCH V3 0/5] Hi: Message-ID: <20190107172517-mutt-send-email-mst@kernel.org> References: <20181229124656.3900-1-jasowang@redhat.com> <20190102154038-mutt-send-email-mst@kernel.org> <0efd115a-a7fb-54bf-5376-59d047a15fd3@redhat.com> <20190106221832-mutt-send-email-mst@kernel.org> <20190106230224-mutt-send-email-mst@kernel.org> <20190107084853-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Mon, 07 Jan 2019 22:25:29 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 07, 2019 at 01:39:15PM -0800, Dan Williams wrote: > On Mon, Jan 7, 2019 at 6:11 AM Michael S. Tsirkin wrote: > > > > On Sun, Jan 06, 2019 at 11:15:20PM -0800, Dan Williams wrote: > > > On Sun, Jan 6, 2019 at 8:17 PM Michael S. Tsirkin wrote: > > > > > > > > On Mon, Jan 07, 2019 at 11:53:41AM +0800, Jason Wang wrote: > > > > > > > > > > On 2019/1/7 上午11:28, Michael S. Tsirkin wrote: > > > > > > On Mon, Jan 07, 2019 at 10:19:03AM +0800, Jason Wang wrote: > > > > > > > On 2019/1/3 上午4:47, Michael S. Tsirkin wrote: > > > > > > > > On Sat, Dec 29, 2018 at 08:46:51PM +0800, Jason Wang wrote: > > > > > > > > > This series tries to access virtqueue metadata through kernel virtual > > > > > > > > > address instead of copy_user() friends since they had too much > > > > > > > > > overheads like checks, spec barriers or even hardware feature > > > > > > > > > toggling. > > > > > > > > Will review, thanks! > > > > > > > > One questions that comes to mind is whether it's all about bypassing > > > > > > > > stac/clac. Could you please include a performance comparison with > > > > > > > > nosmap? > > > > > > > > > > > > > > > On machine without SMAP (Sandy Bridge): > > > > > > > > > > > > > > Before: 4.8Mpps > > > > > > > > > > > > > > After: 5.2Mpps > > > > > > OK so would you say it's really unsafe versus safe accesses? > > > > > > Or would you say it's just a better written code? > > > > > > > > > > > > > > > It's the effect of removing speculation barrier. > > > > > > > > > > > > You mean __uaccess_begin_nospec introduced by > > > > commit 304ec1b050310548db33063e567123fae8fd0301 > > > > ? > > > > > > > > So fundamentally we do access_ok checks when supplying > > > > the memory table to the kernel thread, and we should > > > > do the spec barrier there. > > > > > > > > Then we can just create and use a variant of uaccess macros that does > > > > not include the barrier? > > > > > > > > Or, how about moving the barrier into access_ok? > > > > This way repeated accesses with a single access_ok get a bit faster. > > > > CC Dan Williams on this idea. > > > > > > It would be interesting to see how expensive re-doing the address > > > limit check is compared to the speculation barrier. I.e. just switch > > > vhost_get_user() to use get_user() rather than __get_user(). That will > > > sanitize the pointer in the speculative path without a barrier. > > > > Hmm it's way cheaper even though IIRC it's measureable. > > Jason, would you like to try? > > Although frankly __get_user being slower than get_user feels very wrong. > > Not yet sure what to do exactly but would you agree? > > Agree. __get_user() being faster than get_user() defeats the whole > point of converting code paths to the access_ok() + __get_user() > pattern. Did you mean the reverse? > > > > > > > I recall we had a convert access_ok() discussion with this result here: > > > > > > https://lkml.org/lkml/2018/1/17/929 > > > > Sorry let me try to clarify. IIUC speculating access_ok once > > is harmless. As Linus said the problem is with "_subsequent_ > > accesses that can then be used to perturb the cache". > > > > Thus: > > > > 1. if (!access_ok) > > 2. return > > 3. get_user > > 4. if (!access_ok) > > 5. return > > 6. get_user > > > > Your proposal that Linus nacked was to effectively add a barrier after > > lines 2 and 5 (also using the array_index_nospec trick for speed), > > right? Unfortunately that needs a big API change. > > > > I am asking about adding barrier_nospec within access_ok. > > Thus effectively before lines 1 and 4. > > access_ok will be slower but after all the point of access_ok is > > to then access the same memory multiple times. > > If the barrier is before lines 1 and 4 then it offers no real > protection as far I can see. It will start speculating again on the > user controlled pointer value after the barrier resolves. > > > So we should be making __get_user faster and access_ok slower ... > > I agree, but then the barrier always needs to be after the access_ok() > check unconditionally called in the return path from access_ok(). At > that point it's back to the implementation that Linus nak'd, or I'm > missing some other detail. > > ...but maybe if it is limited to a new access_ok_nospec() then the > concern is addressed? Then rename current __get_user() to > __get_user_nospec() and make a new __get_user() that is back to being > optimal.