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=-2.4 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID, 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 86C21C4646D for ; Mon, 6 Aug 2018 15:24:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3E81A21A53 for ; Mon, 6 Aug 2018 15:24:15 +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="UgTKrWMa" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3E81A21A53 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732798AbeHFRds (ORCPT ); Mon, 6 Aug 2018 13:33:48 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:36550 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729697AbeHFRds (ORCPT ); Mon, 6 Aug 2018 13:33:48 -0400 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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=JZ6dhqCURmkBHO5OW8EF+5TQpTi64JLNMS7WB9UErDA=; b=UgTKrWMafVoE8544+zAl9AQ7Z VddcyMk4+euzyXQLfXZL5IRrEy8+CuGnzntAjaqnxocFmK2fju4nljLzwbNu0cJtLBYyxqRWp8u9V ZwAb7mli0a9lq9uSpEULiPbNiwcFIbFzJYN+oWsdOZ+PMgPt+fHHTC2fM5eiBSxUJODpGYpONn6U/ HpMREghflwHXq/+WUr6zT43m67BvVdVlxTW5x8wqYTT+SmlowArRh9lqPq/kZyPtxqxso2whUc8p2 1vSJzqAQN4d3A/Smi4ejlN93H45JDkDeOX17tN1O/FdTDywL19B3mar7//I8KmQEUfvWpO+8dktId fdlQs6hUQ==; Received: from hch by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1fmhMc-0007bs-Lk; Mon, 06 Aug 2018 15:24:06 +0000 Date: Mon, 6 Aug 2018 08:24:06 -0700 From: Christoph Hellwig To: "Michael S. Tsirkin" Cc: Anshuman Khandual , Benjamin Herrenschmidt , robh@kernel.org, srikar@linux.vnet.ibm.com, aik@ozlabs.ru, Jason Wang , linuxram@us.ibm.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, hch@infradead.org, paulus@samba.org, joe@perches.com, david@gibson.dropbear.id.au, linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net, haren@linux.vnet.ibm.com Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices Message-ID: <20180806152406.GA29020@infradead.org> References: <20180720035941.6844-1-khandual@linux.vnet.ibm.com> <20180802235332-mutt-send-email-mst@kernel.org> <20180803220812-mutt-send-email-mst@kernel.org> <01c74680c4b3aa25d9b4375a9ab5e10046b7c71b.camel@kernel.crashing.org> <20180805032355-mutt-send-email-mst@kernel.org> <74a1e1b8-81e0-84db-6d0d-d8bd9caebb4a@linux.vnet.ibm.com> <20180806163440-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180806163440-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.9.2 (2017-12-15) X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 06, 2018 at 04:36:43PM +0300, Michael S. Tsirkin wrote: > On Mon, Aug 06, 2018 at 02:32:28PM +0530, Anshuman Khandual wrote: > > On 08/05/2018 05:54 AM, Michael S. Tsirkin wrote: > > > On Fri, Aug 03, 2018 at 08:21:26PM -0500, Benjamin Herrenschmidt wrote: > > >> On Fri, 2018-08-03 at 22:08 +0300, Michael S. Tsirkin wrote: > > >>>>>> Please go through these patches and review whether this approach broadly > > >>>>>> makes sense. I will appreciate suggestions, inputs, comments regarding > > >>>>>> the patches or the approach in general. Thank you. > > >>>>> > > >>>>> Jason did some work on profiling this. Unfortunately he reports > > >>>>> about 4% extra overhead from this switch on x86 with no vIOMMU. > > >>>> > > >>>> The test is rather simple, just run pktgen (pktgen_sample01_simple.sh) in > > >>>> guest and measure PPS on tap on host. > > >>>> > > >>>> Thanks > > >>> > > >>> Could you supply host configuration involved please? > > >> > > >> I wonder how much of that could be caused by Spectre mitigations > > >> blowing up indirect function calls... > > >> > > >> Cheers, > > >> Ben. > > > > > > I won't be surprised. If yes I suggested a way to mitigate the overhead. > > > > Did we get better results (lower regression due to indirect calls) with > > the suggested mitigation ? Just curious. > > I'm referring to this: > I wonder whether we can support map_sg and friends being NULL, then use > that when mapping is an identity. A conditional branch there is likely > very cheap. > > I don't think anyone tried implementing this yes. I've done something very similar in the thread I posted a few years ago. I plan to get a version of that upstream for 4.20, but it won't cover the virtio case, just the real direct mapping.