From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44589) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VZ1uf-0004Iq-99 for qemu-devel@nongnu.org; Wed, 23 Oct 2013 13:08:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1VZ1uZ-0003ay-5U for qemu-devel@nongnu.org; Wed, 23 Oct 2013 13:08:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56052) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VZ1uY-0003ap-Th for qemu-devel@nongnu.org; Wed, 23 Oct 2013 13:07:59 -0400 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r9NH7v3C020882 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Wed, 23 Oct 2013 13:07:57 -0400 Message-ID: <5268026A.8080201@redhat.com> Date: Wed, 23 Oct 2013 19:07:54 +0200 From: Max Reitz MIME-Version: 1.0 References: <1382298755-21383-1-git-send-email-mreitz@redhat.com> <52659071.4070003@redhat.com> In-Reply-To: <52659071.4070003@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] qcow2: Unset zero_beyond_eof in save_vmstate List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eric Blake , qemu-devel@nongnu.org Cc: Kevin Wolf , Stefan Hajnoczi On 2013-10-21 22:37, Eric Blake wrote: > On 10/20/2013 08:52 PM, Max Reitz wrote: >> Saving the VM state is done using bdrv_pwrite. This function may perform >> a read-modify-write, which in this case results in data being read from >> beyond the end of the virtual disk. Since we are actually trying to >> access an area which is not a part of the virtual disk, zero_beyond_eof >> has to be set to false before performing the partial write, otherwise >> the VM state may become corrupted. >> >> Signed-off-by: Max Reitz >> --- >> Follow-up to (depends on): >> - qcow2: Restore total_sectors value in save_vmstate > Reviewed-by: Eric Blake > > Do you have test cases that demonstrate the corruption pre-patch? I could write a test case for the other patch, but for this one it would probably be rather difficult. What I did to bisect the bug was just starting a VM over and over while saving a snapshot at some time during boot-up and trying to load that snapshot again. Sometimes, qemu itself would report a corrupted VM state, but most of the time, the guest simply hang or paniced. This is something I can detect interactively, but I don't know if I could write a test for this (at least not for hanging). Max