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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_MUTT 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 A01EDC43387 for ; Sat, 5 Jan 2019 00:33:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7692621871 for ; Sat, 5 Jan 2019 00:33:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726310AbfAEAdk (ORCPT ); Fri, 4 Jan 2019 19:33:40 -0500 Received: from mga11.intel.com ([192.55.52.93]:43548 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725862AbfAEAdk (ORCPT ); Fri, 4 Jan 2019 19:33:40 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga006.fm.intel.com ([10.253.24.20]) by fmsmga102.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 04 Jan 2019 16:33:40 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.56,440,1539673200"; d="scan'208";a="307618368" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.154]) by fmsmga006.fm.intel.com with ESMTP; 04 Jan 2019 16:33:39 -0800 Date: Fri, 4 Jan 2019 16:33:39 -0800 From: Sean Christopherson To: "Michael S. Tsirkin" Cc: Jason Wang , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, davem@davemloft.net Subject: Re: [RFC PATCH V3 1/5] vhost: generalize adding used elem Message-ID: <20190105003339.GE11288@linux.intel.com> References: <20181229124656.3900-1-jasowang@redhat.com> <20181229124656.3900-2-jasowang@redhat.com> <20190104162857-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190104162857-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 04, 2019 at 04:29:34PM -0500, Michael S. Tsirkin wrote: > On Sat, Dec 29, 2018 at 08:46:52PM +0800, Jason Wang wrote: > > Use one generic vhost_copy_to_user() instead of two dedicated > > accessor. This will simplify the conversion to fine grain > > accessors. About 2% improvement of PPS were seen during vitio-user > > txonly test. > > > > Signed-off-by: Jason Wang > > I don't hve a problem with this patch but do you have > any idea how come removing what's supposed to be > an optimization speeds things up? With SMAP, the 2x vhost_put_user() will also mean an extra STAC/CLAC pair, which is probably slower than the overhead of CALL+RET to whatever flavor of copy_user_generic() gets used. CALL+RET is really the only overhead since all variants of copy_user_generic() unroll accesses smaller than 64 bytes, e.g. on a 64-bit system, __copy_to_user() will write all 8 bytes in a single MOV. Removing the special casing also eliminates a few hundred bytes of code as well as the need for hardware to predict count==1 vs. count>1. > > > --- > > drivers/vhost/vhost.c | 11 +---------- > > 1 file changed, 1 insertion(+), 10 deletions(-) > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > index 55e5aa662ad5..f179b5ee14c4 100644 > > --- a/drivers/vhost/vhost.c > > +++ b/drivers/vhost/vhost.c > > @@ -2174,16 +2174,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq, > > > > start = vq->last_used_idx & (vq->num - 1); > > used = vq->used->ring + start; > > - if (count == 1) { > > - if (vhost_put_user(vq, heads[0].id, &used->id)) { > > - vq_err(vq, "Failed to write used id"); > > - return -EFAULT; > > - } > > - if (vhost_put_user(vq, heads[0].len, &used->len)) { > > - vq_err(vq, "Failed to write used len"); > > - return -EFAULT; > > - } > > - } else if (vhost_copy_to_user(vq, used, heads, count * sizeof *used)) { > > + if (vhost_copy_to_user(vq, used, heads, count * sizeof *used)) { > > vq_err(vq, "Failed to write used"); > > return -EFAULT; > > } > > -- > > 2.17.1