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 E2AC1ECAAD3 for ; Wed, 7 Sep 2022 07:59:17 +0000 (UTC) Received: from localhost ([::1]:44380 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1oVpxs-0001Em-Gf for qemu-devel@archiver.kernel.org; Wed, 07 Sep 2022 03:59:16 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:35664) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1oVpwd-0000YL-RF for qemu-devel@nongnu.org; Wed, 07 Sep 2022 03:57:59 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:29754) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1oVpwb-000367-24 for qemu-devel@nongnu.org; Wed, 07 Sep 2022 03:57:58 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1662537474; 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:in-reply-to:in-reply-to: references:references; bh=2A7pM8z/KhzHmoG46+xBmRsLQRtsuvvaO5NdQhHRkBo=; b=TD4KkeXokY3F8311Et0QsyxR1lFxcXkEd+ENZvtwevrWZQAyE1gh5z/ySywMMAPdTIv5Nd tIqxlBhE72Pe46+kS+GJxz5b5iRrlZtc7VoApNVSdnvXZjErebXR1DFAB/POEtIGLibPSf qWtizFl986cq/890pLUCed8r9dKhM84= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-25-d3OzDvmVMOOpPpOTSDZ-kw-1; Wed, 07 Sep 2022 03:57:52 -0400 X-MC-Unique: d3OzDvmVMOOpPpOTSDZ-kw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id A533A801231 for ; Wed, 7 Sep 2022 07:57:52 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.61]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C7C1940CF916; Wed, 7 Sep 2022 07:57:51 +0000 (UTC) Date: Wed, 7 Sep 2022 08:57:49 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: John Snow Cc: Markus Armbruster , Eric Blake , qemu-devel Subject: Re: Maximum QMP reply size Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.2.6 (2022-06-05) X-Scanned-By: MIMEDefang 2.84 on 10.11.54.1 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: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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_DNSWL_LOW=-0.7, 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 Tue, Sep 06, 2022 at 03:38:54PM -0400, John Snow wrote: > Hi, I suspect I have asked this before, but I didn't write it down in > a comment, so I forget my justification... > > In the QMP lib, we need to set a buffering limit for how big a QMP > message can be -- In practice, I found that the largest possible > response was the QAPI schema reply, and I set the code to this: > > # Maximum allowable size of read buffer > _limit = (64 * 1024) > > However, I didn't document if this was a reasonable limit or just a > "worksforme" one. I assume that there's no hard limit for the protocol > or the implementation thereof in QEMU. Is there any kind of value here > that would be more sensible than another? As a reference, libvirt arbitrarily chose 10 MB as the QMP reply limit. It is huge enough it'll be hard to make a QMP reply exceed that, but also still tiny in the context of managing VMs on a host with GB's of RAM. NB, this doesn't mean we allocate 10 MB every time, it is just an upper bound - we only allocate what we actually need. 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 :|