From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:e::133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 41khJp0c48zF3HK for ; Tue, 7 Aug 2018 01:24:25 +1000 (AEST) 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 In-Reply-To: <20180806163440-mutt-send-email-mst@kernel.org> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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.