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=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,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 8128FC432BE for ; Wed, 1 Sep 2021 16:06:39 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 180846103A for ; Wed, 1 Sep 2021 16:06:39 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 180846103A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=nongnu.org Received: from localhost ([::1]:58170 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mLSl3-0003Tj-Uk for qemu-devel@archiver.kernel.org; Wed, 01 Sep 2021 12:06:37 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:58124) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mLSeN-0002fA-8y for qemu-devel@nongnu.org; Wed, 01 Sep 2021 11:59:50 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:32512) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mLSeJ-0004B1-S6 for qemu-devel@nongnu.org; Wed, 01 Sep 2021 11:59:42 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1630511979; h=from:from:reply-to: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=sjFT+L7XFx/8tTCmCVPqYdu3qoIZOrjkklKSYwmj6zo=; b=gSbVu6hMQIfr3yF3ApI76HCS6VWCzH3BfPwkqhmlTxPbRUecBu5lv01CCCX5vOKlwSBbVP C2CclC8QNDacKc3msNr6xiDUx2TJD7uMKEFprEbwi9U/u8Ycr2Uyj2dmX3h/xxrLT+dE8E NaFKnj73Ij+sXyNe484es/hXsSPHEm0= 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-218-y8RDwf8wNhW_yCeBw7JdAQ-1; Wed, 01 Sep 2021 11:59:38 -0400 X-MC-Unique: y8RDwf8wNhW_yCeBw7JdAQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 251FA108592C; Wed, 1 Sep 2021 15:59:37 +0000 (UTC) Received: from redhat.com (unknown [10.39.193.162]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C5DF95D6AD; Wed, 1 Sep 2021 15:59:22 +0000 (UTC) Date: Wed, 1 Sep 2021 16:59:20 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Subject: Re: [PATCH v1 2/3] io: Add zerocopy and errqueue Message-ID: References: <20210831110238.299458-1-leobras@redhat.com> <20210831110238.299458-3-leobras@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.392, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Elena Ufimtseva , John G Johnson , Jagannathan Raman , qemu-block@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , qemu-devel@nongnu.org, Leonardo Bras , Paolo Bonzini , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Fam Zheng Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Sep 01, 2021 at 11:52:13AM -0400, Peter Xu wrote: > On Wed, Sep 01, 2021 at 09:50:56AM +0100, Daniel P. Berrangé wrote: > > On Tue, Aug 31, 2021 at 04:27:04PM -0400, Peter Xu wrote: > > > On Tue, Aug 31, 2021 at 01:57:33PM +0100, Daniel P. Berrangé wrote: > > > > On Tue, Aug 31, 2021 at 08:02:38AM -0300, Leonardo Bras wrote: > > > > > MSG_ZEROCOPY is a feature that enables copy avoidance in TCP/UDP socket > > > > > send calls. It does so by avoiding copying user data into kernel buffers. > > > > > > > > > > To make it work, three steps are needed: > > > > > 1 - A setsockopt() system call, enabling SO_ZEROCOPY > > > > > 2 - Passing down the MSG_ZEROCOPY flag for each send*() syscall > > > > > 3 - Process the socket's error queue, dealing with any error > > > > > > > > AFAICT, this is missing the single most critical aspect of MSG_ZEROCOPY. > > > > > > > > It is non-obvious, but setting the MSG_ZEROCOPY flag turns sendmsg() > > > > from a synchronous call to an asynchronous call. > > > > > > > > It is forbidden to overwrite/reuse/free the buffer passed to sendmsg > > > > until an asynchronous completion notification has been received from > > > > the socket error queue. These notifications are not required to > > > > arrive in-order, even for a TCP stream, because the kernel hangs on > > > > to the buffer if a re-transmit is needed. > > > > > > > > https://www.kernel.org/doc/html/v5.4/networking/msg_zerocopy.html > > > > > > > > "Page pinning also changes system call semantics. It temporarily > > > > shares the buffer between process and network stack. Unlike with > > > > copying, the process cannot immediately overwrite the buffer > > > > after system call return without possibly modifying the data in > > > > flight. Kernel integrity is not affected, but a buggy program > > > > can possibly corrupt its own data stream." > > > > > > > > AFAICT, the design added in this patch does not provide any way > > > > to honour these requirements around buffer lifetime. > > > > > > > > I can't see how we can introduce MSG_ZEROCOPY in any seemless > > > > way. The buffer lifetime requirements imply need for an API > > > > design that is fundamentally different for asynchronous usage, > > > > with a callback to notify when the write has finished/failed. > > > > > > Regarding buffer reuse - it indeed has a very deep implication on the buffer > > > being available and it's not obvious at all. Just to mention that the initial > > > user of this work will make sure all zero copy buffers will be guest pages only > > > (as it's only used in multi-fd), so they should always be there during the > > > process. > > > > That is not the case when migration is using TLS, because the buffers > > transmitted are the ciphertext buffer, not the plaintext guest page. > > Agreed. > > > > > > In short, we may just want to at least having a way to make sure all zero > > > copied buffers are finished using and they're sent after some function returns > > > (e.g., qio_channel_flush()). That may require us to do some accounting on when > > > we called sendmsg(MSG_ZEROCOPY), meanwhile we should need to read out the > > > ee_data field within SO_EE_ORIGIN_ZEROCOPY msg when we do recvmsg() for the > > > error queue and keep those information somewhere too. > > > > > > Some other side notes that reached my mind.. > > > > > > The qio_channel_writev_full() may not be suitable for async operations, as the > > > name "full" implies synchronous to me. So maybe we can add a new helper for > > > zero copy on the channel? > > > > All the APIs that exist today are fundamentally only suitable for sync > > operations. Supporting async correctly will definitely a brand new APIs > > separate from what exists today. > > Yes. What I wanted to say is maybe we can still keep the io_writev() interface > untouched, but just add a new interface at qio_channel_writev_full() level. > > IOW, we could comment on io_writev() that it can be either sync or async now, > just like sendmsg() has that implication too with different flag passed in. > When calling io_writev() with different upper helpers, QIO channel could > identify what flag to pass over to io_writev(). I don't think we should overload any of the existing methods with extra parameters for async. Introduce a completely new set of methods exclusively for this alternative interaction model. I can kinda understand why they took the approach they did with sendmsg, but I wouldn't use it as design motivation for QEMU (except as example of what not to do). Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|