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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 7FDBEC433E0 for ; Wed, 3 Jun 2020 06:23:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5AF3220678 for ; Wed, 3 Jun 2020 06:23:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="AKoUcit5" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726119AbgFCGXT (ORCPT ); Wed, 3 Jun 2020 02:23:19 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:49695 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725878AbgFCGXS (ORCPT ); Wed, 3 Jun 2020 02:23:18 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1591165397; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=lfjOXsWWEkBdc5PrrjtOTiVJWBB/Cmo/FhE9Ax069a4=; b=AKoUcit5GqkRogJEJSn5XhBwBI+tibubMyzMeE2Xe4BYZjxgrvKO5O3Gx/pytTDpYHSaUN Lfu2nSDiDra2+QBd9STTKFk6JV3G23kSspzFFRX13+4n2VqIxU2cZptTGZqaj1bxzNE11F tG4hu26d9pu/HYEWy/EvZF31cu9LvIA= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-338-wGU6dBYQN5WU97h8OzbzCQ-1; Wed, 03 Jun 2020 02:23:15 -0400 X-MC-Unique: wGU6dBYQN5WU97h8OzbzCQ-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id C47AF185695D; Wed, 3 Jun 2020 06:23:14 +0000 (UTC) Received: from [10.72.12.214] (ovpn-12-214.pek2.redhat.com [10.72.12.214]) by smtp.corp.redhat.com (Postfix) with ESMTP id A74F760C81; Wed, 3 Jun 2020 06:23:10 +0000 (UTC) Subject: Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok() To: "Michael S. Tsirkin" Cc: Al Viro , linux-kernel@vger.kernel.org, netdev@vger.kernel.org References: <20200602084257.134555-1-mst@redhat.com> <20200603014815.GR23230@ZenIV.linux.org.uk> <3358ae96-abb6-6be9-346a-0e971cb84dcd@redhat.com> <20200603041849.GT23230@ZenIV.linux.org.uk> <3e723db8-0d55-fae6-288e-9d95905592db@redhat.com> <20200603013600-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Wed, 3 Jun 2020 14:23:08 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <20200603013600-mutt-send-email-mst@kernel.org> 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 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020/6/3 下午1:46, Michael S. Tsirkin wrote: > On Wed, Jun 03, 2020 at 01:18:54PM +0800, Jason Wang wrote: >> On 2020/6/3 下午12:18, Al Viro wrote: >>> On Wed, Jun 03, 2020 at 11:57:11AM +0800, Jason Wang wrote: >>> >>>>> How widely do you hope to stretch the user_access areas, anyway? >>>> To have best performance for small packets like 64B, if possible, we want to >>>> disable STAC not only for the metadata access done by vhost accessors but >>>> also the data access via iov iterator. >>> If you want to try and convince Linus to go for that, make sure to Cc >>> me on that thread. Always liked quality flame... >>> >>> The same goes for interval tree lookups with uaccess allowed. IOW, I _really_ >>> doubt that it's a good idea. >> >> I see. We are just seeking an approach to perform better in order to compete >> with userspace dpdk backends. >> >> I tried another approach of using direct mapping + mmu notifier [1] but the >> synchronization with MMU notifier is not easy to perform well. >> >> [1] https://patchwork.kernel.org/patch/11133009/ >> >> >>>>> Incidentally, who had come up with the name __vhost_get_user? >>>>> Makes for lovey WTF moment for readers - esp. in vhost_put_user()... >>>> I think the confusion comes since it does not accept userspace pointer (when >>>> IOTLB is enabled). >>>> >>>> How about renaming it as vhost_read()/vhost_write() ? >>> Huh? >>> >>> __vhost_get_user() is IOTLB remapping of userland pointer. It does not access >>> userland memory. Neither for read, nor for write. It is used by vhost_get_user() >>> and vhost_put_user(). >>> >>> Why would you want to rename it into vhost_read _or_ vhost_write, and in any case, >>> how do you give one function two names? IDGI... >> >> I get you know, I thought you're concerning the names of >> vhost_get_user()/vhost_put_user() but actually __vhost_get_user(). >> >> Maybe something like __vhost_fetch_uaddr() is better. >> >> Thanks > > It's basically vhost_translate_uaddr isn't it? Yes. > > BTW now I re-read it I don't understand __vhost_get_user_slow: > > > static void __user *__vhost_get_user_slow(struct vhost_virtqueue *vq, > void __user *addr, unsigned int size, > int type) > { > int ret; > > ret = translate_desc(vq, (u64)(uintptr_t)addr, size, vq->iotlb_iov, > ARRAY_SIZE(vq->iotlb_iov), > VHOST_ACCESS_RO); > > .. > } > > how does this work? how can we cast a pointer to guest address without > adding any offsets? I'm not sure I get you here. What kind of offset did you mean? Thanks > >