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 A285CC4332F for ; Wed, 1 Nov 2023 09:27:37 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qy7V3-0003QI-5V; Wed, 01 Nov 2023 05:26:57 -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 1qy7V1-0003Q9-9G for qemu-devel@nongnu.org; Wed, 01 Nov 2023 05:26:55 -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 1qy7Uz-0003H4-Df for qemu-devel@nongnu.org; Wed, 01 Nov 2023 05:26:55 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698830811; 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=LPq5yD+ijPCVp7YSiiKKU2rLz6dLz5cn4zujG/wdHOg=; b=VuoZfN4QtRsbR3sHUU8PGJalcF144VZzx8lE1selz4s44VL0AbaiepLHzU/iEe51Or0iHe EwxJiNetTcdGJjPISYwLiUZV/wdRJIUth+ab3vlCY5IQkGrt3xDFoQYvhRlgi2lgn2Ee83 2HXERMcBgxZn+i6VUFygNF60iqAIU8U= 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-369-CkCSctp5ObeyLEVW0YmNkg-1; Wed, 01 Nov 2023 05:26:49 -0400 X-MC-Unique: CkCSctp5ObeyLEVW0YmNkg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (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 7B96585A58B; Wed, 1 Nov 2023 09:26:49 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.47]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5721C1C060BA; Wed, 1 Nov 2023 09:26:48 +0000 (UTC) Date: Wed, 1 Nov 2023 09:26:46 +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 In-Reply-To: User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.7 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 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 in the context of guest RAM sizes, so no one is going to notice the padding holes in the file from this. IOW, the 1 MB choice is an arbitrary, but somewhat informed choice. 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 :|