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 87585C71130 for ; Tue, 10 Jun 2025 16:56:26 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uP28k-00057a-Su; Tue, 10 Jun 2025 12:47:59 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uP1LO-0001Q2-QC for qemu-devel@nongnu.org; Tue, 10 Jun 2025 11:56:58 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uP1LJ-0008Bk-8C for qemu-devel@nongnu.org; Tue, 10 Jun 2025 11:56:58 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1749571011; 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=mMhPAuXRHNwaFwWidIKGtMPeUCkcvA0svjUZfXoeofo=; b=BPOi0Si5uZlyy+8pVIBXi5gOeIJpHb0rVw3hyQAFvCLDm4dJFrs1RSJqNNmW8AY2BA0sc9 N57+RAnUylx9za0MnsuJceNVhcwUsUYjQ1TmU6iqBRPlskcbVUjAokGBVe6jGVfVsMOwq2 WpVe9DV6u2Ja1zT2a+bMXA3kKfqRWx8= Received: from mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-608-eiwta7BMMu6GtLFiepudcA-1; Tue, 10 Jun 2025 11:56:43 -0400 X-MC-Unique: eiwta7BMMu6GtLFiepudcA-1 X-Mimecast-MFC-AGG-ID: eiwta7BMMu6GtLFiepudcA_1749570997 Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 2CAE319560B2; Tue, 10 Jun 2025 15:56:36 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.87]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E682219560B2; Tue, 10 Jun 2025 15:56:32 +0000 (UTC) Date: Tue, 10 Jun 2025 16:56:29 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Cc: qemu-devel@nongnu.org, "Michael S. Tsirkin" , Peter Maydell , qemu-block@nongnu.org, Stefan Hajnoczi , Kevin Wolf Subject: Re: [PATCH 00/31] Skip automatic zero-init of large arrays / structs in I/O paths Message-ID: References: <20250610123709.835102-1-berrange@redhat.com> <50405e77-e665-4772-9715-3901730d00fd@linaro.org> <4d03fe23-86a1-4030-b5fc-8de8be461770@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4d03fe23-86a1-4030-b5fc-8de8be461770@linaro.org> User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 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: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=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: 2.1.29 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-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Jun 10, 2025 at 05:00:43PM +0200, Philippe Mathieu-Daudé wrote: > On 10/6/25 14:56, Daniel P. Berrangé wrote: > > On Tue, Jun 10, 2025 at 02:49:02PM +0200, Philippe Mathieu-Daudé wrote: > > > On 10/6/25 14:36, Daniel P. Berrangé wrote: > > > > This series is an extension of Stefan's proposal: > > > > > > > > https://lists.nongnu.org/archive/html/qemu-devel/2025-06/msg00736.html > > > > > > > > It used '-Wframe-larger-than=4096' to identify all code locations > > > > with more than 4k on the stack. Any locations in the I/O paths > > > > were chosen to avoid automatic zero-init, to eliminate the performance > > > > overhead of the automatic initialization. > > > > > > Should we eventually add it to our default CFLAGS? > > > > You mean as a way to impose a hard limit on stack frame size ? > > Yes, we could use -Wframe-larger-than=50000 for this release, then > aim to reduce by 5/10k every release until some reasonable limit... I guess the key thing is to define approximately what our target should be ? There are some large stack frames in QEMU device I/O paths that are not unreasonable to have, because they avoid need to malloc in the I/O path. A greater many devices have 4/8k buffers for copying data around, and some bigger ones are fairly valid to have too. On the flip side there's still way too much use of PATH_MAX that is gratuitous and should be removed, but its hard to enforce the latter without negatively impacting the former. 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 :|