Re: [Qemu-devel] [Qemu-block] [RFC PATCH 0/8] qemu-img convert with copy offloading

[Paolo Bonzini]

On 04/04/2018 15:49, Fam Zheng wrote:
>> I posted concerns about the bdrv_co_map_range() interface.  It would be
>> safer to only have a copy_range() interface without exposing how data is
>> mapped outside the driver where race conditions can occur and the format
>> driver no longer has full control over file layout.
> It's a good point, but I couldn't think of a way to implement copy_range 
> between
> two format drivers: both of them need to recurse down to their bs->file and 
> what
> we eventually want is a copy_file_range() on two fds (or an iscsi equivalent):
> 
>     src[qcow2]           ->              dst[raw]
>         |                                   |
>         v                                   v
>     src[file]            ->              dst[file]
>         |                                   |
>         v                                   v
>        fd1               ->                 fd2
>                     copy_file_range
> 
> Maybe we should add BlockDriver.bdrv_co_map_range_{prepare,commit,abort} and
> call them from bdrv_co_copy_range(). This way the code path works pretty much
> the same way to .bdrv_co_preadv/pwritev.

Or you could have bdrv_co_copy_range verify the mapping and return
-EAGAIN if it is not valid anymore?  (Then bdrv_co_map_range should fill
in a BlockDriverMapping struct, or something like that).

Paolo