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 9C1FCC4332F for ; Wed, 1 Nov 2023 14:29:27 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qyCD4-0001Xp-S9; Wed, 01 Nov 2023 10:28:42 -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 1qyCD3-0001Xg-Sz for qemu-devel@nongnu.org; Wed, 01 Nov 2023 10:28:41 -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 1qyCD2-0001AZ-3u for qemu-devel@nongnu.org; Wed, 01 Nov 2023 10:28:41 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698848919; 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=zWSV/baR/jgyz4s6PSIwqUQYi+bhdAf4Mrot8xKf/Lg=; b=BsD4eX9RzZVQ/gR0UMLTW234Ap5T0xuUZkEu/cxSO5LR30zvv1c1cuu7ib2g39K9oe8oWY 4Q3G9+r2RLJWxia8w8rhUopALbn/DZy9EUVyOl8KFzizRhPcrxwmYSYv5eI7NBYqA6PDEi V1YlErbb7UWI5ZX83umh9QxzJl6eNwo= 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-294-Rt14GNf9PhGw6ZFVomohYg-1; Wed, 01 Nov 2023 10:28:27 -0400 X-MC-Unique: Rt14GNf9PhGw6ZFVomohYg-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 528A881F443; Wed, 1 Nov 2023 14:28:27 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.47]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1F93C2026D4C; Wed, 1 Nov 2023 14:28:25 +0000 (UTC) Date: Wed, 1 Nov 2023 14:28:24 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: Fabiano Rosas , qemu-devel@nongnu.org, armbru@redhat.com, Juan Quintela , Leonardo Bras , Claudio Fontana , Nikolay Borisov Subject: Re: [PATCH v2 16/29] migration/ram: Add support for 'fixed-ram' migration restore Message-ID: References: <20231023203608.26370-1-farosas@suse.de> <20231023203608.26370-17-farosas@suse.de> <87r0lieqxm.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.393, 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 Wed, Nov 01, 2023 at 10:21:07AM -0400, Peter Xu wrote: > On Wed, Nov 01, 2023 at 09:26:46AM +0000, Daniel P. Berrangé wrote: > > On Tue, Oct 31, 2023 at 03:03:50PM -0400, Peter Xu wrote: > > > On Wed, Oct 25, 2023 at 11:07:33AM -0300, Fabiano Rosas wrote: > > > > >> +static int parse_ramblock_fixed_ram(QEMUFile *f, RAMBlock *block, ram_addr_t length) > > > > >> +{ > > > > >> + g_autofree unsigned long *bitmap = NULL; > > > > >> + struct FixedRamHeader header; > > > > >> + size_t bitmap_size; > > > > >> + long num_pages; > > > > >> + int ret = 0; > > > > >> + > > > > >> + ret = fixed_ram_read_header(f, &header); > > > > >> + if (ret < 0) { > > > > >> + error_report("Error reading fixed-ram header"); > > > > >> + return -EINVAL; > > > > >> + } > > > > >> + > > > > >> + block->pages_offset = header.pages_offset; > > > > > > > > > > Do you think it is worth sanity checking that 'pages_offset' is aligned > > > > > in some way. > > > > > > > > > > It is nice that we have flexibility to change the alignment in future > > > > > if we find 1 MB is not optimal, so I wouldn't want to force 1MB align > > > > > check htere. Perhaps we could at least sanity check for alignment at > > > > > TARGET_PAGE_SIZE, to detect a gross data corruption problem ? > > > > > > > > > > > > > I don't see why not. I'll add it. > > > > > > Is there any explanation on why that 1MB offset, and how the number is > > > chosen? Thanks, > > > > The fixed-ram format is anticipating the use of O_DIRECT. > > > > With O_DIRECT both the buffers in memory, and the file handle offset > > have alignment requirements. The buffer alignments are usually page > > sized, and QEMU RAM blocks will trivially satisfy those. > > > > The file handle offset alignment varies per filesystem. While you can > > query the alignment for the FS holding the file with statx(), that is > > not appropriate todo. If a user saves/restores QEMU state to file, we > > must assume there is a chance the user will copy the saved state to a > > different filesystem. > > > > IOW, we want alignment to satisfy the likely worst case. > > > > Picking 1 MB is a nice round number that is large enough that it is > > almost certainly going to satisfy any filesystem alignment. In fact > > it is likely massive overkill. None the less 1 MB is also still tiny > > Is that calculated by something like max of possible host (small) page > sizes? I've no idea what's it for all archs, the max small page size I'm > aware of is 64K, but I don't know a lot archs. It wasn't anything as precise as that. It is literally just "1MB" looks large enough that we don't need to spend time to investigate per arch page sizes. Having said that I'm now having slight self-doubt wrt huge pages, though I swear we investigated it last year when first discussing this feature. The guest memory will of course already be suitably aligned, but I'm wondering if the filesystem I/O places any offset alignment constraints related to non-default page size. 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 :|