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 A6BBFC4332F for ; Tue, 31 Oct 2023 09:04:24 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qxkf3-00046h-T8; Tue, 31 Oct 2023 05:03:45 -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 1qxkez-0003ew-Eq for qemu-devel@nongnu.org; Tue, 31 Oct 2023 05:03:42 -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 1qxkew-0007Dv-Eh for qemu-devel@nongnu.org; Tue, 31 Oct 2023 05:03:41 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698743017; 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=psjah80H3K7eQBOfyGElxTAM0/FLjk8+5yfZ3LuV8EQ=; b=eqYmQ2xxNeRZAMEKs/ac5GjEXdhxfXFvZUDib2qBPdAz8qLEk8Sns61sgZbTkEBJHHzfUe fcgkd/fyUnq1qibjQhqzaozSMH/s5hbAC6etUpz9Ispt15+NgZyGKTXhcWzzY932pV7Wn+ MoFEpD7HLLhAvlA9PFEZh2tcITF8pQ0= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-168-BCDiUP6hMg69CwFLSpDWPw-1; Tue, 31 Oct 2023 05:03:34 -0400 X-MC-Unique: BCDiUP6hMg69CwFLSpDWPw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (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 mimecast-mx02.redhat.com (Postfix) with ESMTPS id 944D43C1042E; Tue, 31 Oct 2023 09:03:33 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.55]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 088121121306; Tue, 31 Oct 2023 09:03:31 +0000 (UTC) Date: Tue, 31 Oct 2023 09:03:29 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Fabiano Rosas Cc: Markus Armbruster , qemu-devel@nongnu.org, Juan Quintela , Peter Xu , Leonardo Bras , Claudio Fontana , Eric Blake Subject: Re: [PATCH v2 28/29] migration: Add direct-io parameter Message-ID: References: <20231023203608.26370-1-farosas@suse.de> <20231023203608.26370-29-farosas@suse.de> <878r7svapt.fsf@pond.sub.org> <87msw7ddfp.fsf@suse.de> <87cyx2epsv.fsf@suse.de> <87cywvenbd.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87cywvenbd.fsf@suse.de> User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.3 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: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.483, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.29 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 Mon, Oct 30, 2023 at 07:51:34PM -0300, Fabiano Rosas wrote: > I could use some advice on how to solve this situation. The fdset code > at monitor/fds.c and the add-fd command don't seem to be usable outside > the original use-case of passing fds with different open flags. > > There are several problems, the biggest one being that there's no way to > manipulate the set of file descriptors aside from asking for duplication > of an fd that matches a particular set of flags. > > That doesn't work for us because the two fds we need (one for main > channel, other for secondary channels) will have the same open flags. So > the fdset code will always return the first one it finds in the set. QEMU may want multiple FDs *internally*, but IMHO that fact should not be exposed to mgmt applications. It would be valid for a QEMU impl to share the same FD across multiple threads, or have a different FD for each thread. All threads are using pread/pwrite, so it is safe for them to use the same FD if they desire. It is a private impl choice for QEMU at any given point in time and could change over time. Thus from the POV of the mgmt app, QEMU is writing to a single file, no matter how many threads are involved & thus it should only need to supply a single FD for thta file. QEMU can then call 'dup()' on that FD as many times as it desires, and use fcntl() to toggle O_DIRECT if and when it needs to. > Another problem (or feature) of the fdset code is that it only removes > an fd when qmp_remove_fd() is called if the VM runstate is RUNNING, > which means that the migration code cannot remove the fds after use > reliably. We need to be able to remove them to make sure we use the > correct fds in a subsequent migration. The "easy" option is to just add a new API that does what you want. Maybe during review someone will then point out why the orgianl API works the way it does. 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 :|