Re: [Qemu-devel] [PATCH] qemu-img rebase: Fix for undersized backing files

[Kevin Wolf <kwolf@redhat.com>]

Am 07.12.2011 13:06, schrieb Stefan Hajnoczi:
> On Wed, Dec 7, 2011 at 12:02 PM, Kevin Wolf <kwolf@redhat.com> wrote:
>> Am 07.12.2011 12:50, schrieb Stefan Hajnoczi:
>>> On Wed, Dec 7, 2011 at 11:45 AM, Kevin Wolf <kwolf@redhat.com> wrote:
>>>> Backing files may be smaller than the corresponding COW file. When
>>>> reading directly from the backing file, qemu-img rebase must consider
>>>> this and assume zero sectors after the end of backing files.
>>>>
>>>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
>>>> ---
>>>>  qemu-img.c |   42 +++++++++++++++++++++++++++++++++---------
>>>>  1 files changed, 33 insertions(+), 9 deletions(-)
>>>
>>> Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
>>>
>>> It almost feels like we want a bdrv_read() variation that does the
>>> zeroing beyond end of image instead of duplicating this.
>>
>> Almost. If we have a third user, it's probably worth it.
>>
>> Actually, if we make a bdrv_co_readv_backing() instead of a synchronous
>> one, qcow2 would be the third user (and maybe VMDK could use it, too).
>> That would require moving qemu-img into a coroutine first, which I think
>> you wanted to do anyway?
> 
> If we can make qemu-img the last user of synchronous interfaces then
> it would be nice to convert it.  Right now I'm happy that we're
> solidifying coroutines and converting synchronous hardware emulation,
> hopefully we'll get to the point where there is no real reason to keep
> the synchronous API around anymore.

The point is, if I were to introduce a bdrv_co_readv_backing() function,
I certainly wouldn't like to add all the bdrv_(aio_)read/write emulation
code for it, too.

Kevin