[Bug 1860759] Re: [REGRESSION] option `-snapshot` ignored with blockdev

[Max Reitz]

Hi,

I don’t know much about libvirt, but I would have thought that any
manual modification of the qemu command line isn’t supported and might
always break.

Anyway, from a QEMU POV, -snapshot only works with -drive (this includes
-hda, etc.).  It doesn’t work with -blockdev.  I can see that this isn’t
documented for -snapshot, but basically whenever -blockdev is used, the
user assumes full responsibility for the block graph (or at least that
particular subgraph).  We cannot enable snapshot functionality then.

So this can’t be fixed in qemu, as -snapshot doesn’t make sense for
-blockdev.  This behavior should be documented, though.

As for libvirt, I don’t know.  I would be surprised if it had a
guarantee for keeping manual qemu command line additions working, and I
can’t imagine that it would scan the XML for “legacy” qemu parameters
and interpret them itself (which it would need to do to keep -snapshot
working for -blockdev).

Max

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Title:
  [REGRESSION] option `-snapshot` ignored with blockdev

Status in QEMU:
  New

Bug description:
  After upgrade of qemu 3.1.0 → 4.2.0 I found that running with libvirt doesn't 
honor `-snapshot` option anymore. I.e. disk images get modified.
  Using `-hda` option honors `-snapshot`

  So I made a test case without libvirt. Testcase using 4.2.0:

  > qemu -hda tmp-16G.img -cdrom regular-rescue-latest-x86_64.iso -m 2G

  This works fine and tmp-16G.img stays unmodified.

  But:
  > /usr/bin/qemu-system-x86_64 -name guest=test-linux,debug-threads=on -S 
-machine pc-i440fx-3.1,accel=kvm,usb=off,vmport=off,dump-guest-core=off -cpu 
Broadwell-noTSX,vme=on,ss=on,f16c=on,rdrand=on,hypervisor=on,arat=on,tsc-adjust=on,xsaveopt=on,pdpe1gb=on,abm=on
 -m 2048 -overcommit mem-lock=off -smp 3,sockets=3,cores=1,threads=1 -uuid 
d32a9191-f51d-4fae-a419-b73d85b49198 -no-user-config -nodefaults -rtc 
base=utc,driftfix=slew -global kvm-pit.lost_tick_policy=delay -no-hpet 
-no-shutdown -global PIIX4_PM.disable_s3=1 -global PIIX4_PM.disable_s4=1 -boot 
strict=on -device ich9-usb-ehci1,id=usb,bus=pci.0,addr=0x5.0x7 -device 
ich9-usb-uhci1,masterbus=usb.0,firstport=0,bus=pci.0,multifunction=on,addr=0x5 
-device ich9-usb-uhci2,masterbus=usb.0,firstport=2,bus=pci.0,addr=0x5.0x1 
-device ich9-usb-uhci3,masterbus=usb.0,firstport=4,bus=pci.0,addr=0x5.0x2 
-blockdev 
\{\"driver\":\"file\",\"filename\":\"/tmp/regular-rescue-latest-x86_64.iso\",\"node-name\":\"libvirt-2-storage\",\"auto-read-only\":true,\"discard\":\"unmap\"}
 -blockdev 
\{\"node-name\":\"libvirt-2-format\",\"read-only\":true,\"driver\":\"raw\",\"file\":\"libvirt-2-storage\"}
 -device ide-cd,bus=ide.0,unit=0,drive=libvirt-2-format,id=ide0-0-0,bootindex=1 
-blockdev 
\{\"driver\":\"file\",\"filename\":\"/tmp/tmp-2G.img\",\"node-name\":\"libvirt-1-storage\",\"auto-read-only\":true,\"discard\":\"unmap\"}
 -blockdev 
\{\"node-name\":\"libvirt-1-format\",\"read-only\":false,\"driver\":\"qcow2\",\"file\":\"libvirt-1-storage\",\"backing\":null}
 -device 
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x7,drive=libvirt-1-format,id=virtio-disk0
 -netdev user,id=hostnet0 -device 
e1000,netdev=hostnet0,id=net0,mac=52:54:00:ab:d8:29,bus=pci.0,addr=0x3 -chardev 
pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -device 
qxl-vga,id=video0,ram_size=67108864,vram_size=67108864,vram64_size_mb=0,vgamem_mb=16,max_outputs=1,bus=pci.0,addr=0x2
 -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device 
hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -device 
virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6 -snapshot -sandbox 
on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -msg 
timestamp=on

  This modifies tmp-16G.img.

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