From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:49827) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Ui2Td-0006DO-LG for qemu-devel@nongnu.org; Thu, 30 May 2013 09:01:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Ui2TZ-0007YR-35 for qemu-devel@nongnu.org; Thu, 30 May 2013 09:01:09 -0400 Message-ID: <51A74D79.7040204@redhat.com> Date: Thu, 30 May 2013 15:00:41 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <51A7036A.3050407@ozlabs.ru> <51A7049F.6040207@redhat.com> <51A70B3D.90609@ozlabs.ru> <51A71705.6060009@kamp.de> In-Reply-To: <51A71705.6060009@kamp.de> 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: Peter Lieven Cc: Alexey Kardashevskiy , "qemu-ppc@nongnu.org" , "qemu-devel@nongnu.org" , David Gibson 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. Paolo