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Bug 735553

Summary: libvirt should check for ABI compatibility of snapshots
Product: Red Hat Enterprise Linux 6 Reporter: Eric Blake <eblake>
Component: libvirtAssignee: Eric Blake <eblake>
Status: CLOSED ERRATA QA Contact: Virtualization Bugs <virt-bugs>
Severity: unspecified Docs Contact:
Priority: unspecified    
Version: 6.2CC: ajia, dyuan, mzhan, nzhang, rwu, veillard, whuang
Target Milestone: rc   
Target Release: ---   
Hardware: Unspecified   
OS: Unspecified   
Whiteboard:
Fixed In Version: libvirt-0.9.4-12.el6 Doc Type: Bug Fix
Doc Text:
Story Points: ---
Clone Of: Environment:
Last Closed: 2011-12-06 11:28:21 UTC Type: ---
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:
Bug Depends On:    
Bug Blocks: 638510, 743047    

Description Eric Blake 2011-09-03 13:54:35 UTC 
Description of problem:
Libvirt is not maintaining enough information about snapshots, and risks corrupting guests that have done any hot plug or other configuration changes between the time a snapshot is created then later reverted to.

Version-Release number of selected component (if applicable):
libvirt-0.9.4-7.el6

How reproducible:
100%

Steps to Reproduce:
1. virsh start dom
2. virsh snapshot-create-as dom snap
3. virsh snapshot-dumpxml dom snap
4. virsh shutdown dom
5. virsh setmaxmem dom $(larger value)
6. virsh start dom
7. virsh snapshot-revert dom snap
  
Actual results:
in step 2, libvirt saves only the domain uuid in the snapshot.  As a result, in step 7, libvirt has no way of knowing that the snapshot was tied to a different amount of guest RAM, tries to revert to the snapshot using a monitor command while keeping a single qemu process, and the difference in memory sizes makes things go haywire

Expected results:
libvirt should have tracked the full domain xml (visible in step 3), and used that in step 7 to either reject the revert (due to incompatible ABI) or to stop the one qemu process, and start another one with the former memory sizing.

Additional info:
Upstream patches to reject things based on incompatible ABI:
https://www.redhat.com/archives/libvir-list/2011-September/msg00137.html
Upstream plan to add a VIR_DOMAIN_SNAPSHOT_REVERT_FORCE flag to allow starting a new qemu (note that restarting qemu is not seamless, which is why it should require a flag):
https://www.redhat.com/archives/libvir-list/2011-August/msg00361.html
Comment 1 Eric Blake 2011-09-03 13:56:44 UTC 
Getting this fixed, at least to the point of recognizing incompatible ABI, is a prereq to bug 638510 support for live snapshots via the snapshot_blkdev qemu monitor command.  Upstream does not yet have a solution for VIR_DOMAIN_SNAPSHOT_REVERT_FORCE implemented, so depending on time constraints, we may have to split that half into a separate BZ for later enhancement.
Comment 2 Eric Blake 2011-09-04 01:41:53 UTC 
In POST:
http://post-office.corp.redhat.com/archives/rhvirt-patches/2011-September/msg00135.html
Comment 5 Nan Zhang 2011-09-13 12:18:41 UTC 
Got an error when revert from created snapshot, move back to ASSIGNED.

# virsh start foo
Domain foo started

# virsh snapshot-create-as foo snap
Domain snapshot snap created

# virsh snapshot-dumpxml foo snap
<domainsnapshot>
  <name>snap</name>
  <state>running</state>
  <creationTime>1315915439</creationTime>
<domain type='kvm'>
  <name>foo</name>
  <uuid>d3782e1f-e34a-bf7c-414b-4f74d3837e96</uuid>
  <memory>524288</memory>
  <currentMemory>524288</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64' machine='rhel6.1.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='writeback'/>
      <source file='/var/lib/libvirt/images/foo.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/boot/cdrom.img'/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='1' unit='0'/>
    </disk>
    <controller type='ide' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <controller type='fdc' index='0'/>
    <interface type='bridge'>
      <mac address='52:54:00:85:15:ce'/>
      <source bridge='rhevm'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </interface>
    <serial type='pty'>
      <target port='0'/>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
    <input type='tablet' bus='usb'/>
    <input type='mouse' bus='ps2'/>
    <graphics type='spice' autoport='yes'>
      <image compression='auto_glz'/>
      <jpeg compression='auto'/>
      <zlib compression='auto'/>
      <playback compression='on'/>
    </graphics>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </memballoon>
  </devices>
</domain>
</domainsnapshot>

# virsh destroy foo
Domain foo destroyed

# virsh setmaxmem foo 1048576

# virsh start foo
Domain foo started

# virsh snapshot-revert foo snap
error: operation failed: Error -22 while loading VM state
Comment 6 Eric Blake 2011-09-13 20:30:11 UTC 
Aha - the ABI compatibility checking was incomplete, and we happened to trip on one of the few places where the change is guest-visible but the code didn't flag it.  Proposed this upstream:
https://www.redhat.com/archives/libvir-list/2011-September/msg00471.html
Comment 7 Eric Blake 2011-09-14 16:22:51 UTC 
Back in POST:
http://post-office.corp.redhat.com/archives/rhvirt-patches/2011-September/msg00458.html
Comment 9 Nan Zhang 2011-09-22 07:51:20 UTC 
Tested with libvirt-0.9.4-12.el6.x86_64, already been fixed. Move to VERIFIED.

[root@localhost ~]# virsh start foo
Domain foo started

[root@localhost ~]# virsh snapshot-create-as foo snap
Domain snapshot snap created

[root@localhost ~]# virsh snapshot-list foo
 Name                 Creation Time             State
------------------------------------------------------------
 snap                 2010-02-08 20:58:16 -0500 running

[root@localhost ~]# virsh snapshot-dumpxml foo snap
<domainsnapshot>
  <name>snap</name>
  <state>running</state>
  <creationTime>1265680696</creationTime>
<domain type='kvm'>
  <name>foo</name>
  <uuid>5035868c-6f23-63d1-d05c-1fc5ff963134</uuid>
  <memory>524288</memory>
  <currentMemory>524288</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64' machine='rhel6.2.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/var/lib/libvirt/images/foo.img'/>
      <target dev='vda' bus='ide'/>
      <address type='drive' controller='0' bus='0' unit='0'/>
    </disk>
    <controller type='virtio-serial' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
    </controller>
    <controller type='ide' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <interface type='network'>
      <mac address='52:54:00:11:df:7f'/>
      <source network='default'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
    <serial type='pty'>
      <target port='0'/>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
    <channel type='pty'>
      <target type='virtio' name='org.linux-kvm.port.0'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>
    <input type='tablet' bus='usb'/>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes'/>
    <sound model='ich6'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </sound>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </memballoon>
  </devices>
</domain>
</domainsnapshot>

[root@localhost ~]# virsh destroy foo
Domain foo destroyed

[root@localhost ~]# virsh setmaxmem foo 1048576

[root@localhost ~]# virsh start foo
Domain foo started

[root@localhost ~]# virsh snapshot-revert foo snap
error: unsupported configuration: Target domain max memory 524288 does not match source 1048576
Comment 10 errata-xmlrpc 2011-12-06 11:28:21 UTC 
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2011-1513.html