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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=unavailable 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 503B0C282C4 for ; Thu, 24 Jan 2019 04:11:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1F2062184C for ; Thu, 24 Jan 2019 04:11:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727222AbfAXELh (ORCPT ); Wed, 23 Jan 2019 23:11:37 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58694 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726313AbfAXELh (ORCPT ); Wed, 23 Jan 2019 23:11:37 -0500 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 513187D0F1; Thu, 24 Jan 2019 04:11:36 +0000 (UTC) Received: from [10.72.12.164] (ovpn-12-164.pek2.redhat.com [10.72.12.164]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 041B710021B1; Thu, 24 Jan 2019 04:11:30 +0000 (UTC) Subject: Re: [PATCH net-next V4 5/5] vhost: access vq metadata through kernel virtual address From: Jason Wang To: "Michael S. Tsirkin" Cc: virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org References: <20190123095557.30168-1-jasowang@redhat.com> <20190123095557.30168-6-jasowang@redhat.com> <20190123085821-mutt-send-email-mst@kernel.org> <335ba55b-087f-4b35-6311-540070b9647f@redhat.com> Message-ID: <4521d3d8-561e-53f5-98e1-bf7ace003701@redhat.com> Date: Thu, 24 Jan 2019 12:11:28 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <335ba55b-087f-4b35-6311-540070b9647f@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Thu, 24 Jan 2019 04:11:36 +0000 (UTC) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2019/1/24 下午12:07, Jason Wang wrote: > > On 2019/1/23 下午10:08, Michael S. Tsirkin wrote: >> On Wed, Jan 23, 2019 at 05:55:57PM +0800, Jason Wang wrote: >>> It was noticed that the copy_user() friends that was used to access >>> virtqueue metdata tends to be very expensive for dataplane >>> implementation like vhost since it involves lots of software checks, >>> speculation barrier, hardware feature toggling (e.g SMAP). The >>> extra cost will be more obvious when transferring small packets since >>> the time spent on metadata accessing become more significant. >>> >>> This patch tries to eliminate those overheads by accessing them >>> through kernel virtual address by vmap(). To make the pages can be >>> migrated, instead of pinning them through GUP, we use MMU notifiers to >>> invalidate vmaps and re-establish vmaps during each round of metadata >>> prefetching if necessary. For devices that doesn't use metadata >>> prefetching, the memory accessors fallback to normal copy_user() >>> implementation gracefully. The invalidation was synchronized with >>> datapath through vq mutex, and in order to avoid hold vq mutex during >>> range checking, MMU notifier was teared down when trying to modify vq >>> metadata. >>> >>> Another thing is kernel lacks efficient solution for tracking dirty >>> pages by vmap(), this will lead issues if vhost is using file backed >>> memory which needs care of writeback. This patch solves this issue by >>> just skipping the vma that is file backed and fallback to normal >>> copy_user() friends. This might introduce some overheads for file >>> backed users but consider this use case is rare we could do >>> optimizations on top. >>> >>> Note that this was only done when device IOTLB is not enabled. We >>> could use similar method to optimize it in the future. >>> >>> Tests shows at most about 22% improvement on TX PPS when using >>> virtio-user + vhost_net + xdp1 + TAP on 2.6GHz Broadwell: >>> >>>          SMAP on | SMAP off >>> Before: 5.0Mpps | 6.6Mpps >>> After:  6.1Mpps | 7.4Mpps >>> >>> Signed-off-by: Jason Wang >> >> So this is the bulk of the change. >> Threee things that I need to look into >> - Are there any security issues with bypassing the speculation barrier >>    that is normally present after access_ok? > > > If we can make sure the bypassing was only used in a kthread (vhost), > it should be fine I think. > > >> - How hard does the special handling for >>    file backed storage make testing? > > > It's as simple as un-commenting vhost_can_vmap()? Or I can try to hack > qemu or dpdk to test this. > > >>    On the one hand we could add a module parameter to >>    force copy to/from user. on the other that's >>    another configuration we need to support. > > > That sounds sub-optimal since it leave the choice to users. > > >>    But iotlb is not using vmap, so maybe that's enough >>    for testing. >> - How hard is it to figure out which mode uses which code. It's as simple as tracing __get_user() usage in vhost process? Thanks >> >> >> >> Meanwhile, could you pls post data comparing this last patch with the >> below?  This removes the speculation barrier replacing it with a >> (useless but at least more lightweight) data dependency. > > > SMAP off > > Your patch: 7.2MPPs > > vmap: 7.4Mpps > > I don't test SMAP on, since it will be much slow for sure. > > Thanks > > >> >> Thanks! >> >> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >> index bac939af8dbb..352ee7e14476 100644 >> --- a/drivers/vhost/vhost.c >> +++ b/drivers/vhost/vhost.c >> @@ -739,7 +739,7 @@ static int vhost_copy_to_user(struct >> vhost_virtqueue *vq, void __user *to, >>       int ret; >>         if (!vq->iotlb) >> -        return __copy_to_user(to, from, size); >> +        return copy_to_user(to, from, size); >>       else { >>           /* This function should be called after iotlb >>            * prefetch, which means we're sure that all vq >> @@ -752,7 +752,7 @@ static int vhost_copy_to_user(struct >> vhost_virtqueue *vq, void __user *to, >>                        VHOST_ADDR_USED); >>             if (uaddr) >> -            return __copy_to_user(uaddr, from, size); >> +            return copy_to_user(uaddr, from, size); >>             ret = translate_desc(vq, (u64)(uintptr_t)to, size, >> vq->iotlb_iov, >>                        ARRAY_SIZE(vq->iotlb_iov), >> @@ -774,7 +774,7 @@ static int vhost_copy_from_user(struct >> vhost_virtqueue *vq, void *to, >>       int ret; >>         if (!vq->iotlb) >> -        return __copy_from_user(to, from, size); >> +        return copy_from_user(to, from, size); >>       else { >>           /* This function should be called after iotlb >>            * prefetch, which means we're sure that vq >> @@ -787,7 +787,7 @@ static int vhost_copy_from_user(struct >> vhost_virtqueue *vq, void *to, >>           struct iov_iter f; >>             if (uaddr) >> -            return __copy_from_user(to, uaddr, size); >> +            return copy_from_user(to, uaddr, size); >>             ret = translate_desc(vq, (u64)(uintptr_t)from, size, >> vq->iotlb_iov, >>                        ARRAY_SIZE(vq->iotlb_iov), >> @@ -855,13 +855,13 @@ static inline void __user >> *__vhost_get_user(struct vhost_virtqueue *vq, >>   ({ \ >>       int ret = -EFAULT; \ >>       if (!vq->iotlb) { \ >> -        ret = __put_user(x, ptr); \ >> +        ret = put_user(x, ptr); \ >>       } else { \ >>           __typeof__(ptr) to = \ >>               (__typeof__(ptr)) __vhost_get_user(vq, ptr,    \ >>                         sizeof(*ptr), VHOST_ADDR_USED); \ >>           if (to != NULL) \ >> -            ret = __put_user(x, to); \ >> +            ret = put_user(x, to); \ >>           else \ >>               ret = -EFAULT;    \ >>       } \ >> @@ -872,14 +872,14 @@ static inline void __user >> *__vhost_get_user(struct vhost_virtqueue *vq, >>   ({ \ >>       int ret; \ >>       if (!vq->iotlb) { \ >> -        ret = __get_user(x, ptr); \ >> +        ret = get_user(x, ptr); \ >>       } else { \ >>           __typeof__(ptr) from = \ >>               (__typeof__(ptr)) __vhost_get_user(vq, ptr, \ >>                                  sizeof(*ptr), \ >>                                  type); \ >>           if (from != NULL) \ >> -            ret = __get_user(x, from); \ >> +            ret = get_user(x, from); \ >>           else \ >>               ret = -EFAULT; \ >>       } \