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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 228E7C43334 for ; Tue, 14 Jun 2022 08:40:24 +0000 (UTC) Received: from localhost ([::1]:47518 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1o1263-00062t-83 for qemu-devel@archiver.kernel.org; Tue, 14 Jun 2022 04:40:23 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:35462) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1o122m-00052H-8v for qemu-devel@nongnu.org; Tue, 14 Jun 2022 04:37:00 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:60467) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1o122k-0006rE-6a for qemu-devel@nongnu.org; Tue, 14 Jun 2022 04:36:59 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1655195817; 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=raV/DSUlxirGo9fUvVdNubFktnHZiumx/P9pa4iatpA=; b=M6aj5WcVVuV4olIJE1/yQr5kUxd4b/C2bj9r0xd51YKKwEudvEdHbC7mCgJ15MAGdCANZ3 J7MI/jPgnOX86j09RNOBKFQY9+Fx/QBJLuHlQnY2frIGI1+YXi8XxivQQJW0uZgY5CiH+F pAIElECfNobNsp3ONKGO92Ak+Z+v7Oc= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-21-Q9RQAQstOGm2Coq8sSTe9Q-1; Tue, 14 Jun 2022 04:36:53 -0400 X-MC-Unique: Q9RQAQstOGm2Coq8sSTe9Q-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 53D46299E75A; Tue, 14 Jun 2022 08:36:53 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.137]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 02A1B492C3B; Tue, 14 Jun 2022 08:36:51 +0000 (UTC) Date: Tue, 14 Jun 2022 09:36:49 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Leonardo Bras Soares Passos Cc: Peter Xu , =?utf-8?B?5b6Q6Zev?= , David Gilbert , Juan Quintela , qemu-devel Subject: Re: [PATCH v2 1/2] QIOChannelSocket: Reduce ifdefs to improve readability Message-ID: References: <20220608210403.84006-1-leobras@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.1 (2022-02-19) X-Scanned-By: MIMEDefang 2.85 on 10.11.54.10 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Jun 13, 2022 at 06:21:18PM -0300, Leonardo Bras Soares Passos wrote: > On Fri, Jun 10, 2022 at 5:25 AM Daniel P. Berrangé wrote: > > > > [...] > > > Ok, so if it is checked earlier then we merely need an assert. > > > > if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) { > > #ifdef QEMU_MSG_ZEROCOPY > > sflags = MSG_ZEROCOPY; > > zero_copy_enabled = true; > > #else > > g_assert_unreachable(); > > #endif > > > } > > Ok, I will add that in the next version. > > > > > > > > > > > > @@ -592,15 +594,13 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc, > > > > > return QIO_CHANNEL_ERR_BLOCK; > > > > > case EINTR: > > > > > goto retry; > > > > > -#ifdef QEMU_MSG_ZEROCOPY > > > > > case ENOBUFS: > > > > > - if (sflags & MSG_ZEROCOPY) { > > > > > + if (zero_copy_enabled) { > > > > > > > > if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) > > > > > > > > avoids the #ifdef without needing to add yet another > > > > variable expressing what's already expressed in both > > > > 'flags' and 'sflags'. > > > > > > Yes, it does, but at the cost of not compiling-out the zero-copy part > > > when it's not supported, > > > since the QIO_CHANNEL_WRITE_FLAG_ZERO_COPY comes as a parameter. This ends up > > > meaning there will be at least one extra test for every time this > > > function is called (the one in the next patch). > > > > The cost of a simple bit test is between negligible-and-non-existant > > with branch prediction. I doubt it would be possible to even measure > > it. > > Yeah, you are probably right on that. > So the main learning point here is that it's not worth creating a new > boolean for compiling-out > code that should not impact performance ? As ever "it depends" so there's no hard rule, and sometimes it can verge on bikeshed colouring :-) I didn't like the variable in this case, because it introduces a 3rd variable to the method for representing whether zero copy is need, which is excessive. I'm not a fan of redundancy as it can often then lead to inconsistency. So it would need a compelling reason why it is better, which is difficult for such a simple method. If the code was more complex, a variable might have benefit of clarity, but in this case IMHO it was just overkill. With 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 :|