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 A8E45C4167B for ; Tue, 31 Oct 2023 13:46:40 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qxp3s-0000yW-Td; Tue, 31 Oct 2023 09:45:41 -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 1qxp3q-0000yK-RB for qemu-devel@nongnu.org; Tue, 31 Oct 2023 09:45:38 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qxp3n-0006mE-MC for qemu-devel@nongnu.org; Tue, 31 Oct 2023 09:45:38 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698759932; 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=uQWDpR4TinU24LtQ2KGlB4XgMsMBqc2kEHFduddV71s=; b=h7b8aFIqEDvTBWUQJsCPwOniDMsS0Vdbx4I6ixp6/q0VoE+4QnsjH2A0DvZNj1xjfu4fLs mk2M94pkGttY5ZJW8zWDtZhlGyrkXYLfrbvGdj8X+5q79a6VDfQNg5YjA5bylWtPFbu6XW 3OTXfIbpuKyp5tay3xzpGUxXkXtSXhE= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-479-hXE-DBKTNVyqkCz4-7iFtw-1; Tue, 31 Oct 2023 09:45:29 -0400 X-MC-Unique: hXE-DBKTNVyqkCz4-7iFtw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (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 9D468831501; Tue, 31 Oct 2023 13:45:28 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.55]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 4E81E40C6EB9; Tue, 31 Oct 2023 13:45:27 +0000 (UTC) Date: Tue, 31 Oct 2023 13:45:25 +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> <878r7jdjrf.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <878r7jdjrf.fsf@suse.de> User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.2 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: -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_H4=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 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 ? > > > 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. > > Ok, so I think the way to go here is for QEMU to receive a file + offset > instead of an FD. That way QEMU can have adequate control of the > resources for the migration. I don't remember why we went on the FD > tangent. Is it not acceptable for libvirt to provide the file name + > offset? FD passing means QEMU does not need privileges to open the file which could be useful. 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 :|