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 774A2C4332F for ; Tue, 31 Oct 2023 15:24:23 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qxqaL-0001OZ-0F; Tue, 31 Oct 2023 11:23:17 -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 1qxqaJ-0001O0-1J for qemu-devel@nongnu.org; Tue, 31 Oct 2023 11:23:15 -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 1qxqaH-0000Ya-64 for qemu-devel@nongnu.org; Tue, 31 Oct 2023 11:23:14 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698765790; 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=V0LeFM5hxhBXUCYD++t+ECZ0OntMAcqB16zdfe7y99o=; b=Evk6x9gtydg/8vbGgo2OzWkM0r8DIcoKNkZ/enh7PGpO8Kg+rIpa5quwYD3LxgXmmfs/7B QYRnFTtMo2Ru96uIeqorUS0OZmMoDP2ADEnqmj401ezxyeuuqW027EDrAAro2TdYVu2Nwq APQMneyF4g9UAetml0+m6KQb92yITSQ= 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-46-CtOkN51UN_-JGvkigN97wQ-1; Tue, 31 Oct 2023 11:23:07 -0400 X-MC-Unique: CtOkN51UN_-JGvkigN97wQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (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 905FF3C1E9DB; Tue, 31 Oct 2023 15:22:57 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.55]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5D67B2026D4C; Tue, 31 Oct 2023 15:22:56 +0000 (UTC) Date: Tue, 31 Oct 2023 15:22:54 +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: <878r7svapt.fsf@pond.sub.org> <87msw7ddfp.fsf@suse.de> <87cyx2epsv.fsf@suse.de> <87cywvenbd.fsf@suse.de> <878r7jdjrf.fsf@suse.de> <875y2meua3.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <875y2meua3.fsf@suse.de> User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.4 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=-0.481, 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, 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-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Oct 31, 2023 at 11:33:24AM -0300, Fabiano Rosas wrote: > Daniel P. Berrangé writes: > > > On Tue, Oct 31, 2023 at 10:05:56AM -0300, Fabiano Rosas wrote: > >> Daniel P. Berrangé writes: > >> > >> > 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. > >> > > >> > >> Sure, I don't disagree. However up until last week we had a seemingly > >> usable "add-fd" command that allows the user to provide a *set of file > >> descriptors* to QEMU. It's just now that we're learning that interface > >> serves only a special use-case. > > > > AFAICT though we don't need add-fd to support passing many files > > for our needs. Saving only requires a single FD. All others can > > be opened by dup(), so the limitation of add-fd is irrelevant > > surely ? > > Only once we decide to use one FD. If we had a generic add-fd backend, > then that's already a user-facing API, so the "implementation detail" > argument becomes weaker. > > With a single FD we'll need to be very careful about what code is > allowed to run while the multifd channels are doing IO. Since O_DIRECT > is not widely supported, now we have to also be careful about someone > using that QEMUFile handle to do unaligned writes and not even noticing > that it breaks direct IO. None of this in unworkable, of course, I just > find the design way clearer with just the file name + offset. I guess I'm not seeing the problem still. A single FD is passed across from libvirt, but QEMU is free to turn that into *many* FDs for its internal use, using dup() and then setting O_DIRECT on as many/few of the dup()d FDs as its wants to. 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 :|