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=-1.0 required=3.0 tests=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 986BCC43381 for ; Tue, 12 Mar 2019 07:53:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7143F214D8 for ; Tue, 12 Mar 2019 07:53:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727349AbfCLHxt (ORCPT ); Tue, 12 Mar 2019 03:53:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42524 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725832AbfCLHxt (ORCPT ); Tue, 12 Mar 2019 03:53:49 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 904B987621; Tue, 12 Mar 2019 07:53:48 +0000 (UTC) Received: from [10.72.12.17] (ovpn-12-17.pek2.redhat.com [10.72.12.17]) by smtp.corp.redhat.com (Postfix) with ESMTP id 86C1260CA3; Tue, 12 Mar 2019 07:53:39 +0000 (UTC) Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() From: Jason Wang To: James Bottomley , 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 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> <1552367685.23859.22.camel@HansenPartnership.com> Message-ID: <7f779c16-58d1-dd4f-54cf-a7538d4b6fe4@redhat.com> Date: Tue, 12 Mar 2019 15:53:37 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Tue, 12 Mar 2019 07:53:48 +0000 (UTC) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2019/3/12 下午3:51, Jason Wang wrote: > > On 2019/3/12 下午1:14, James Bottomley wrote: >> 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. > > > It would be good if kmap()/kunmap() can do this but looks like we can > not assume this? For example, sparc's flush_dcache_page() Sorry, I meant kunmap_atomic(). Thanks > doesn't do flush_dcache_page(). And bio_copy_data_iter() do > flush_dcache_page() after kunmap_atomic(). > > Thanks > > >> >> James >> >