From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40668) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VgZBK-000839-9x for qemu-devel@nongnu.org; Wed, 13 Nov 2013 07:04:32 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1VgZBE-0004QG-3b for qemu-devel@nongnu.org; Wed, 13 Nov 2013 07:04:26 -0500 Received: from mx1.redhat.com ([209.132.183.28]:26257) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VgZBD-0004Oo-Po for qemu-devel@nongnu.org; Wed, 13 Nov 2013 07:04:20 -0500 Date: Wed, 13 Nov 2013 13:04:10 +0100 From: Igor Mammedov Message-ID: <20131113130410.3ff7b082@thinkpad> In-Reply-To: <20131112221039.GA16838@redhat.com> References: <1384264707-9947-1-git-send-email-imammedo@redhat.com> <1384264707-9947-3-git-send-email-imammedo@redhat.com> <528272BA.30402@redhat.com> <20131112221039.GA16838@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: Paolo Bonzini , aliguori@amazon.com, qemu-devel@nongnu.org, afaerber@suse.de, kraxel@redhat.com On Wed, 13 Nov 2013 00:10:39 +0200 "Michael S. Tsirkin" wrote: > On Tue, Nov 12, 2013 at 07:26:02PM +0100, Paolo Bonzini wrote: > > Il 12/11/2013 14:58, Igor Mammedov ha scritto: > > > 'etc/reserved-memory-end' will allow QEMU to tell BIOS where PCI > > > BARs mapping could safely start in high memory. > > > > > > Allowing BIOS to start mapping 64-bit PCI BARs at address where it > > > wouldn't conflict with other mappings QEMU might place before it. > > > > > > That permits QEMU to reserve extra address space before > > > 64-bit PCI hole for memory hotplug. > > > > I may be royally wrong, but I think the new file should only be added to > > new machine types. Otherwise, after migrating old machine types from > > new QEMU to old QEMU, you may end up with PCI BARs mapped outside the > > "PCI windows" that exist until before patch 1/2 of this series. > > > > Does this make sense? > > Yes. > Generally FW CFG must not be added/removed for a given machine types, > otherwise guest that is migrated while reading it will > get a corrupted result: half old and half new. Is it true for a file 'etc/reserved-memory-end' though? I've debugged SeaBIOS to learn more about it, and new->old migration with following reboot, showed that file is not found by SeaBIOS (well since old QEMU doesn't have it), as result SeaBIOS fallbacks to the old behavior placing 64-PCI bars right above ram_over_4G as it was intended. And with 'etc/reserved-memory-end' == ram_over_4G_end as it is in this patch, there isn't issue whatsoever. Looks like there is no migrations issues with files, provided SeaBIOS knows how to deal with a missing file. -- Regards, Igor