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 89523C4332F for ; Wed, 1 Nov 2023 12:17:49 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qyA9X-0007Td-IN; Wed, 01 Nov 2023 08:16:55 -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 1qyA9I-0007Nb-7D for qemu-devel@nongnu.org; Wed, 01 Nov 2023 08:16:44 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1qyA9G-0008Rs-9x for qemu-devel@nongnu.org; Wed, 01 Nov 2023 08:16:39 -0400 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id A966721854; Wed, 1 Nov 2023 12:16:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1698840996; h=from:from:reply-to: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=ViD5gk77ywZD7AtBE4YAuDTAX9UPhVM+8jUdyQujzzU=; b=pql1phq/KQ1f3jxG81gNlUjWRA1MLXrzZ4a3WTYqNQJEw2/a21W6PZ231/8/nRW6YyUVI2 VxH6gTmJSZrPVK9Sj7LG8zy2MoIu8fVV/gGrLU2ZTrebKabNOJsZ5UbsjnmCicN93czhZs T7atCFa1MNvNXHBFCutYuVene1NaTqc= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1698840996; h=from:from:reply-to: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=ViD5gk77ywZD7AtBE4YAuDTAX9UPhVM+8jUdyQujzzU=; b=0XW63fWeQVL4KrbeWTggz0nx1lv7m0p0Wl3Ofkhdz9DXRAXLqt7H4ZGy8DuadmuLii0oUa 6MpkP/nP3JpKaKCQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 3BF0E1348D; Wed, 1 Nov 2023 12:16:36 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id djZ7AqRBQmUTUAAAMHmgww (envelope-from ); Wed, 01 Nov 2023 12:16:36 +0000 From: Fabiano Rosas To: =?utf-8?Q?Daniel_P=2E_Berrang=C3=A9?= 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 In-Reply-To: References: <87cywvenbd.fsf@suse.de> <878r7jdjrf.fsf@suse.de> <875y2meua3.fsf@suse.de> <8734xqeqly.fsf@suse.de> <87wmv2d33p.fsf@suse.de> Date: Wed, 01 Nov 2023 09:16:33 -0300 Message-ID: <87zfzxfz32.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=195.135.220.28; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Daniel P. Berrang=C3=A9 writes: > On Tue, Oct 31, 2023 at 04:05:46PM -0300, Fabiano Rosas wrote: >> Daniel P. Berrang=C3=A9 writes: >>=20 >> > On Tue, Oct 31, 2023 at 12:52:41PM -0300, Fabiano Rosas wrote: >> >> Daniel P. Berrang=C3=A9 writes: >> >> > >> >> > I guess I'm not seeing the problem still. A single FD is passed ac= ross >> >> > 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. >> >>=20 >> >> The problem is that duplicated FDs share the file status flags. If we >> >> set O_DIRECT on the multifd channels and the main thread happens to do >> >> an unaligned write with qemu_file_put* then the filesystem will fail >> >> that write. >> > >> > Doh, I had forgotten that sharing. >> > >> > Do we have any synchronization between multifd channels and the main >> > thread ? eg does the main thread wait for RAM sending completion >> > before carrying on writing other non-RAM data ? >>=20 >> We do have, but the issue with that approach is that there are no rules >> for adding data into the stream. Anyone could add a qemu_put_* call >> right in the middle of the section for whatever reason. >>=20 >> That is almost a separate matter due to our current compatibility model >> being based on capabilities rather than resilience of the stream >> format. So extraneous data in the stream always causes the migration to >> break. >>=20 >> But with the O_DIRECT situation we'd be adding another aspect to >> this. Not only changing the code requires syncing capabilities (as it >> does today), but it would also require knowing which parts of the stream >> can be interrupted by new data and which cannot. >>=20 >> So while it would probably work, it's also a little fragile. If QEMU >> were given 2 FDs or given access to the file, then only the multifd >> channels would get O_DIRECT and they are guaranteed to not have >> extraneous unaligned data showing up. > > So the problem with add-fd is that when requesting a FD, the monitor > code masks flags with O_ACCMODE. What if we extended it such that > the monitor masked with O_ACCMODE | O_DIRECT. > > That would let us pass 1 plain FD and one O_DIRECT fd, and be able > to ask for each separately by setting O_DIRECT or not. That would likely work. The usage gets a little more complicated, but we'd be using fdset as it was intended. Should we keep the direct-io capability? If the user now needs to set O_DIRECT and also set the cap, that seems a little redundant. I could keep O_DIRECT in the flags (when supported) and test after open if we got the flag set. If it's not set, then we remove O_DIRECT from the flags and retry. > Existing users of add-fd are not likely to be affected since none of > them will be using O_DIRECT. I had thought of passing a comparison function into monitor_fdset_dup_fd_add() to avoid affecting existing users. That would require plumbing it through qemu_open_internal() or moving monitor_fdset_dup_fd_add() earlier in the stack (probably more sensible). I'll not worry about that for now though, let's first make sure the approach you suggested works.