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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2DE8FC433FE for ; Thu, 28 Oct 2021 04:31:59 +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 E8F9D6023F for ; Thu, 28 Oct 2021 04:31:58 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org E8F9D6023F 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]:56678 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mfx54-0003iM-13 for qemu-devel@archiver.kernel.org; Thu, 28 Oct 2021 00:31:58 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:48890) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mfx46-0002yT-6B for qemu-devel@nongnu.org; Thu, 28 Oct 2021 00:30:59 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:32699) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mfx43-0000HV-G0 for qemu-devel@nongnu.org; Thu, 28 Oct 2021 00:30:57 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1635395453; 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: in-reply-to:in-reply-to:references:references; bh=DWgnKO8XQNdqYD5kfulurHIYAvwQFg9vtHlA84pPUsA=; b=PTOstugaO9zwf1I3v2XY4aeRfGCI7ReiQfIb2v/e3LC2khB9POlNiJY701UqAYDm4Stxx/ 5/8edTFUUw5KvzwLGQWcrJVD+C6vuLCUDcX6FbkFX7AeY8D9gKU0kELE7vEHe6T15NYAH+ i7trB04lYn1sMwgFBIEo1RzhFN4nM0w= 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-550-KF0_ZwNSMmamNjE2YNQEdA-1; Thu, 28 Oct 2021 00:30:50 -0400 X-MC-Unique: KF0_ZwNSMmamNjE2YNQEdA-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 D81431006AA3 for ; Thu, 28 Oct 2021 04:30:49 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-112-7.ams2.redhat.com [10.36.112.7]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E0C8360CA1; Thu, 28 Oct 2021 04:30:34 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 758C811380A7; Thu, 28 Oct 2021 06:30:33 +0200 (CEST) From: Markus Armbruster To: Leonardo Bras Soares Passos Subject: Re: [PATCH v4 3/3] multifd: Implement zerocopy write in multifd migration (multifd-zerocopy) References: <20211009075612.230283-1-leobras@redhat.com> <20211009075612.230283-4-leobras@redhat.com> <20211011193156.biedorxjetduaf7y@redhat.com> <87sfx6g55m.fsf@dusky.pond.sub.org> Date: Thu, 28 Oct 2021 06:30:33 +0200 In-Reply-To: (Leonardo Bras Soares Passos's message of "Wed, 27 Oct 2021 22:56:51 -0300") Message-ID: <87sfwl3h5i.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@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, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham 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: , Cc: "Daniel P. =?utf-8?Q?Berrang=C3=A9?=" , Juan Quintela , Jason Wang , "Dr. David Alan Gilbert" , Peter Xu , qemu-devel , Eric Blake Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Leonardo Bras Soares Passos writes: [...] >> The general argument for having QAPI schema 'if' mirror the C >> implementation's #if is introspection. Let me explain why that matters. >> >> Consider a management application that supports a range of QEMU >> versions, say 5.0 to 6.2. Say it wants to use an QMP command that is >> new in QEMU 6.2. The sane way to do that is to probe for the command >> with query-qmp-schema. Same for command arguments, and anything else >> QMP. >> >> If you doubt "sane", check out Part II of "QEMU interface introspection: >> From hacks to solutions"[*]. >> >> The same technique works when a QMP command / argument / whatever is >> compile-time conditional ('if' in the schema). The code the management >> application needs anyway to deal with older QEMU now also deals with >> "compiled out". Nice. >> >> Of course, a command or argument present in QEMU can still fail, and the >> management application still needs to handle failure. Distinguishing >> different failure modes can be bothersome and/or fragile. >> >> By making the QAPI schema conditional mirror the C conditional, you >> squash the failure mode "this version of QEMU supports it, but this >> build of QEMU does not" into "this version of QEMU does not support >> it". Makes sense, doesn't it? >> >> A minor additional advantage is less generated code. >> >> >> >> [*] http://events17.linuxfoundation.org/sites/events/files/slides/armbru-qemu-introspection.pdf >> > > This was very informative, thanks! > I now understand the rationale about this choice. > > TBH I am not very used to this syntax. > I did a take a peek at some other json files, and ended adding this > lines in code, which compiled just fine: > > for : enum MigrationParameter > {'name': 'multifd-zerocopy', 'if' : 'CONFIG_LINUX'}, > > for : struct MigrateSetParameters and struct MigrationParameters: > '*multifd-zerocopy': { 'type': 'bool', 'if': 'CONFIG_LINUX' }, > > Is that enough? Is there any other necessary change? Looks good to me. The QAPI schema language is documented in docs/devel/qapi-code-gen.rst. If you're curious, you can diff code generated into qapi/ before and after adding the 'if'. > Thanks for reviewing and for helping out with this! My pleasure!