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=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 EA57FC282C6 for ; Thu, 24 Jan 2019 04:53:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C6D06218A2 for ; Thu, 24 Jan 2019 04:53:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727347AbfAXExt (ORCPT ); Wed, 23 Jan 2019 23:53:49 -0500 Received: from mx1.redhat.com ([209.132.183.28]:55468 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726249AbfAXExt (ORCPT ); Wed, 23 Jan 2019 23:53:49 -0500 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 9DD5B811A9; Thu, 24 Jan 2019 04:53:48 +0000 (UTC) Received: from redhat.com (ovpn-121-100.rdu2.redhat.com [10.10.121.100]) by smtp.corp.redhat.com (Postfix) with ESMTP id A17042657F; Thu, 24 Jan 2019 04:53:47 +0000 (UTC) Date: Wed, 23 Jan 2019 23:53:47 -0500 From: "Michael S. Tsirkin" To: Jason Wang Cc: virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH net-next V4 5/5] vhost: access vq metadata through kernel virtual address Message-ID: <20190123235219-mutt-send-email-mst@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> <4521d3d8-561e-53f5-98e1-bf7ace003701@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4521d3d8-561e-53f5-98e1-bf7ace003701@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Thu, 24 Jan 2019 04:53:48 +0000 (UTC) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Thu, Jan 24, 2019 at 12:11:28PM +0800, Jason Wang wrote: > > 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 Well there are now mtu notifiers etc etc. It's hardly as well contained as that. > > > > > > > > > > > > > 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; \ > > >       } \