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Re: [Qemu-devel] [PATCH] block: change default of .has_zero_init to 0

From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH] block: change default of .has_zero_init to 0
Date: Fri, 28 Jun 2013 10:16:55 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130623 Thunderbird/17.0.7

On 28.06.2013 10:06, Kevin Wolf wrote:
Am 27.06.2013 um 15:52 hat Peter Lieven geschrieben:
.has_zero_init defaults to 1 for all formats and protocols.

this is a dangerous default since this means that all
new added drivers need to manually overwrite it to 0 if
they do not ensure that a device is zero initialized
after bdrv_create().

if a driver needs to explicitly set this value to
1 its easier to verify the correctness in the review process.

during review of the existing drivers it turned out
that ssh and gluster had a wrong default of 1.
both protocols support host_devices as backend
which are not by default zero initialized. this
wrong assumption will lead to possible corruption
if qemu-img convert is used to write to such a backend.

a similar problem with the wrong default existed for
iscsi mose likely because the driver developer did
oversee the default value of 1.

Signed-off-by: Peter Lieven <address@hidden>
  block.c               |    8 +++++++-
  block/qcow.c          |    1 +
  block/qcow2.c         |    1 +
  block/qed.c           |    1 +
  block/raw-posix.c     |   10 +---------
  block/raw-win32.c     |    7 +------
  block/rbd.c           |    1 +
  block/sheepdog.c      |    1 +
  block/vdi.c           |    1 +
  block/vmdk.c          |    1 +
  include/block/block.h |    1 +
  11 files changed, 17 insertions(+), 16 deletions(-)
You forgot cow, which is also a simple case that can be handled in this

vpc is still easy, but a bit more complicated than a constant return 1,
because it depends on the subformat whether a new image will inherit the
has_zero_init property from the underlying storage or whether it always
produces zeros (for VHD_FIXED type images, it's basically raw + footer).
I'll send a separate patch for this.
shall I leave this to the new 0 default until your patch is ready?

A similar situation exists for vmdk, I think, just that the difference
can be per extent there. I guess we need to return 0 if one of the
extents is flat and has an underlying storage returning 0. I'll leave
this part to Fam. For now, please remove the bdrv_has_zero_init_1 for
vmdk from this patch as it's unsafe.
ok I will mention that the value is changed for vmdk (and vpc) in the
commit message.


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