From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:34377) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Ui33t-0005BD-69 for qemu-devel@nongnu.org; Thu, 30 May 2013 09:38:43 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Ui33m-0000dd-GF for qemu-devel@nongnu.org; Thu, 30 May 2013 09:38:37 -0400 Received: from mail-pb0-x235.google.com ([2607:f8b0:400e:c01::235]:44790) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Ui33m-0000cc-AJ for qemu-devel@nongnu.org; Thu, 30 May 2013 09:38:30 -0400 Received: by mail-pb0-f53.google.com with SMTP id un4so359555pbc.40 for ; Thu, 30 May 2013 06:38:28 -0700 (PDT) Message-ID: <51A7564D.5090202@ozlabs.ru> Date: Thu, 30 May 2013 23:38:21 +1000 From: Alexey Kardashevskiy MIME-Version: 1.0 References: <51A7036A.3050407@ozlabs.ru> <51A7049F.6040207@redhat.com> <51A70B3D.90609@ozlabs.ru> <51A71705.6060009@kamp.de> <51A74D79.7040204@redhat.com> In-Reply-To: <51A74D79.7040204@redhat.com> Content-Type: text/plain; charset=KOI8-R Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] broken incoming migration List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: "qemu-ppc@nongnu.org" , Peter Lieven , "qemu-devel@nongnu.org" , David Gibson On 05/30/2013 11:00 PM, Paolo Bonzini wrote: > Il 30/05/2013 11:08, Peter Lieven ha scritto: >> Am 30.05.2013 10:18, schrieb Alexey Kardashevskiy: >>> On 05/30/2013 05:49 PM, Paolo Bonzini wrote: >>>> Il 30/05/2013 09:44, Alexey Kardashevskiy ha scritto: >>>>> Hi! >>>>> >>>>> I found the migration broken on pseries platform, specifically, this patch >>>>> broke it: >>>>> >>>>> f1c72795af573b24a7da5eb52375c9aba8a37972 >>>>> migration: do not sent zero pages in bulk stage >>>>> >>>>> The idea is not to send zero pages to the destination guest which is >>>>> expected to have 100% empty RAM. >>>>> >>>>> However on pseries plaftorm the guest always has some stuff in the RAM as a >>>>> part of initialization (device tree, system firmware and rtas (?)) so it is >>>>> not completely empty. As the source guest cannot detect this, it skips some >>>>> pages during migration and we get a broken destination guest. Bug. >>>>> >>>>> While the idea is ok in general, I do not see any easy way to fix it as >>>>> neither QEMUMachine::init nor QEMUMachine::reset callbacks has information >>>>> about whether we are about to receive a migration or not (-incoming >>>>> parameter) and we cannot move device-tree and system firmware >>>>> initialization anywhere else. >>>>> >>>>> ram_bulk_stage is static and cannot be disabled from the platform >>>>> initialization code. >>>>> >>>>> So what would the community suggest? >>>> Revert the patch. :) >>> I'll wait for 24 hours (forgot to cc: the author) and then post a revert >>> patch :) >>> >> does this problem only occur on pseries emulation? > > Probably not. On a PC, it would occur if you had 4K of zeros in the > source BIOS but not in the destination BIOS. When you reboot, the BIOS > image is wrong. > >> not sending zero pages is not only a performance benefit it also makes >> overcomitted memory usable. the madv_dontneed seems to kick in asynchronously >> and memory is not available immediately. > > You could also scan the page for nonzero values before writing it. Scan where? On the source? It is there already. On the destination? Won't just trying to read a page kill all effect from MADV_DONTNEED? -- Alexey