Re: [Qemu-block] [RFC PATCH for-2.9] block: Ignore guest dev permissions during incoming migration

[Kashyap Chamarthy]

On Tue, Apr 04, 2017 at 05:35:56PM +0200, Kevin Wolf wrote:
> Usually guest devices don't like other writers to the same image, so
> they use blk_set_perm() to prevent this from happening. In the migration
> phase before the VM is actually running, though, they don't have a
> problem with writes to the image. On the other hand, storage migration
> needs to be able to write to the image in this phase, so the restrictive
> blk_set_perm() call of qdev devices breaks it.
> 
> This patch flags all BlockBackends with a qdev device as
> blk->disable_perm during incoming migration, which means that the
> requested permissions are stored in the BlockBackend, but not actually
> applied to its root node yet.
> 
> Once migration has finished and the VM should be resumed, the
> permissions are applied. If they cannot be applied (e.g. because the NBD
> server used for block migration hasn't been shut down), resuming the VM
> fails.
> 
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  block/block-backend.c | 40 +++++++++++++++++++++++++++++++++++++++-
>  include/block/block.h |  2 ++
>  migration/migration.c |  8 ++++++++
>  qmp.c                 |  6 ++++++
>  4 files changed, 55 insertions(+), 1 deletion(-)

With your fix applied, now I don't see the original error ("error:
internal error: unable to execute QEMU command 'nbd-server-add':
Conflicts with use by drive-virtio-disk0 as 'root', which does not allow
'write' on #block163"), and I can export a disk via `nbd-server-add`
with 'writeable' flag.  

However, with this fix, running `drive-mirror` seg-faults source QEMU.

Reproducer:

(1) On destination QEMU

$ /home/stack/build/build-qemu/x86_64-softmmu/qemu-system-x86_64 \
    -display none -nodefconfig -nodefaults -m 512 \
    -blockdev 
node-name=bar,driver=qcow2,file.driver=file,file.filename=./dst-disk.qcow2 \
    -serial unix:/tmp/monitor,server,nowait \
    -incoming tcp:localhost:6666 -qmp stdio
{"QMP": {"version": {"qemu": {"micro": 93, "minor": 8, "major": 2}, "package": 
" (v2.9.0-rc3-3-g3a8624b)"}, "capabilities": []}}
{ "execute": "qmp_capabilities" }
{"return": {}}
{ "execute": "nbd-server-start", "arguments": { "addr": { "type": 
"inet","data": { "host": "localhost", "port": "3333" } } } }
{"return": {}}
{ "execute": "nbd-server-add", "arguments": { "device": "bar","writable": true 
} }
{"return": {}}
/home/stack/src/qemu/nbd/server.c:nbd_receive_request():L706: read failed


(2) On source QEMU:

$  /home/stack/build/build-qemu/x86_64-softmmu/qemu-system-x86_64 \
    -display none -nodefconfig -nodefaults -m 512 \
    -device virtio-scsi-pci,id=scsi -device virtio-serial-pci \
    -blockdev 
node-name=foo,driver=qcow2,file.driver=file,file.filename=./cirros-0.3.5.qcow2
    -qmp stdio
{"QMP": {"version": {"qemu": {"micro": 93, "minor": 8, "major": 2}, "package": 
" (v2.9.0-rc3-3-g3a8624b)"}, "capabilities": []}}
{ "execute": "qmp_capabilities" }
{"return": {}}
{ "execute": "drive-mirror", "arguments": { "device": "foo", "target": 
"nbd:localhost:3333:exportname=bar", "sync": "full","format": "raw", "mode": 
"existing" } }
Segmentation fault (core dumped)


[...]

-- 
/kashyap