From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: XDP performance regression due to CONFIG_RETPOLINE Spectre V2 Date: Thu, 12 Apr 2018 16:56:53 +0200 Message-ID: <20180412145653.GA7172@lst.de> References: <20180412155029.0324fe58@redhat.com> <20180412145123.GA7048@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "xdp-newbies@vger.kernel.org" , "netdev@vger.kernel.org" , Christoph Hellwig , David Woodhouse , William Tu , =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= , "Karlsson, Magnus" , Alexander Duyck , Arnaldo Carvalho de Melo To: Jesper Dangaard Brouer Return-path: Received: from verein.lst.de ([213.95.11.211]:55489 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752763AbeDLO4z (ORCPT ); Thu, 12 Apr 2018 10:56:55 -0400 Content-Disposition: inline In-Reply-To: <20180412145123.GA7048@lst.de> Sender: netdev-owner@vger.kernel.org List-ID: On Thu, Apr 12, 2018 at 04:51:23PM +0200, Christoph Hellwig wrote: > On Thu, Apr 12, 2018 at 03:50:29PM +0200, Jesper Dangaard Brouer wrote: > > --------------- > > Implement support for keeping the DMA mapping through the XDP return > > call, to remove RX map/unmap calls. Implement bulking for XDP > > ndo_xdp_xmit and XDP return frame API. Bulking allows to perform DMA > > bulking via scatter-gatter DMA calls, XDP TX need it for DMA > > map+unmap. The driver RX DMA-sync (to CPU) per packet calls are harder > > to mitigate (via bulk technique). Ask DMA maintainer for a common > > case direct call for swiotlb DMA sync call ;-) > > Why do you even end up in swiotlb code? Once you bounce buffer your > performance is toast anyway.. I guess that is because x86 selects it as the default as soon as we have more than 4G memory. That should be solveable fairly easily with the per-device dma ops, though.