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=-0.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS autolearn=unavailable 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 D788BC43381 for ; Tue, 12 Mar 2019 05:14:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A448D2087C for ; Tue, 12 Mar 2019 05:14:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=hansenpartnership.com header.i=@hansenpartnership.com header.b="xYRhAyEt" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727056AbfCLFOs (ORCPT ); Tue, 12 Mar 2019 01:14:48 -0400 Received: from bedivere.hansenpartnership.com ([66.63.167.143]:59170 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725792AbfCLFOr (ORCPT ); Tue, 12 Mar 2019 01:14:47 -0400 Received: from localhost (localhost [127.0.0.1]) by bedivere.hansenpartnership.com (Postfix) with ESMTP id 13D308EE14F; Mon, 11 Mar 2019 22:14:47 -0700 (PDT) Received: from bedivere.hansenpartnership.com ([127.0.0.1]) by localhost (bedivere.hansenpartnership.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 7WtYXw-QTR3T; Mon, 11 Mar 2019 22:14:46 -0700 (PDT) Received: from [153.66.254.194] (unknown [50.35.68.20]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by bedivere.hansenpartnership.com (Postfix) with ESMTPSA id 3BBB58EE130; Mon, 11 Mar 2019 22:14:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=hansenpartnership.com; s=20151216; t=1552367686; bh=AW3dolA78+lzEd83tz6QQ+NipDZU+vh1dNrlssMHnVU=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=xYRhAyEtbC9eyaeABxwkOllNzY+9kjpj9FhbZaQeAldW3lO5vxwuVmjyIp3KNFqBG LD4cAqF1Nc+zpH6hIwFgFhhJvaNSxmq46EBcnVR1m2djSf239RlCYCBinxR2v67gmY JHvGMLPDK4DrxvhtR46iCuydqs9rTvtKEn+Y8lKk= Message-ID: <1552367685.23859.22.camel@HansenPartnership.com> Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() From: James Bottomley To: Jason Wang , David Miller , mst@redhat.com Cc: hch@infradead.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, peterx@redhat.com, linux-mm@kvack.org, aarcange@redhat.com, linux-arm-kernel@lists.infradead.org, linux-parisc@vger.kernel.org Date: Mon, 11 Mar 2019 22:14:45 -0700 In-Reply-To: <6b6dcc4a-2f08-ba67-0423-35787f3b966c@redhat.com> References: <20190308141220.GA21082@infradead.org> <56374231-7ba7-0227-8d6d-4d968d71b4d6@redhat.com> <20190311095405-mutt-send-email-mst@kernel.org> <20190311.111413.1140896328197448401.davem@davemloft.net> <6b6dcc4a-2f08-ba67-0423-35787f3b966c@redhat.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.26.6 Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2019-03-12 at 10:59 +0800, Jason Wang wrote: > On 2019/3/12 上午2:14, David Miller wrote: > > From: "Michael S. Tsirkin" > > Date: Mon, 11 Mar 2019 09:59:28 -0400 > > > > > On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote: > > > > On 2019/3/8 下午10:12, Christoph Hellwig wrote: > > > > > On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote: > > > > > > This series tries to access virtqueue metadata through > > > > > > kernel virtual > > > > > > address instead of copy_user() friends since they had too > > > > > > much > > > > > > overheads like checks, spec barriers or even hardware > > > > > > feature > > > > > > toggling. This is done through setup kernel address through > > > > > > vmap() and > > > > > > resigter MMU notifier for invalidation. > > > > > > > > > > > > Test shows about 24% improvement on TX PPS. TCP_STREAM > > > > > > doesn't see > > > > > > obvious improvement. > > > > > > > > > > How is this going to work for CPUs with virtually tagged > > > > > caches? > > > > > > > > Anything different that you worry? > > > > > > If caches have virtual tags then kernel and userspace view of > > > memory > > > might not be automatically in sync if they access memory > > > through different virtual addresses. You need to do things like > > > flush_cache_page, probably multiple times. > > > > "flush_dcache_page()" > > > I get this. Then I think the current set_bit_to_user() is suspicious, > we > probably miss a flush_dcache_page() there: > > > static int set_bit_to_user(int nr, void __user *addr) > { > unsigned long log = (unsigned long)addr; > struct page *page; > void *base; > int bit = nr + (log % PAGE_SIZE) * 8; > int r; > > r = get_user_pages_fast(log, 1, 1, &page); > if (r < 0) > return r; > BUG_ON(r != 1); > base = kmap_atomic(page); > set_bit(bit, base); > kunmap_atomic(base); This sequence should be OK. get_user_pages() contains a flush which clears the cache above the user virtual address, so on kmap, the page is coherent at the new alias. On parisc at least, kunmap embodies a flush_dcache_page() which pushes any changes in the cache above the kernel virtual address back to main memory and makes it coherent again for the user alias to pick it up. James