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Re: [libvirt] [Qemu-devel] [PATCH v3] Add support for fd: protocol



On Wed, Jul 27, 2011 at 10:11:06AM +0200, Kevin Wolf wrote:
> Am 26.07.2011 18:57, schrieb Corey Bryant:
> >>>>  diff --git a/block/cow.c b/block/cow.c
> >>>>  index 4cf543c..e17f8e7 100644
> >>>>  --- a/block/cow.c
> >>>>  +++ b/block/cow.c
> >>>>  @@ -82,6 +82,11 @@ static int cow_open(BlockDriverState *bs, int flags)
> >>>>        pstrcpy(bs->backing_file, sizeof(bs->backing_file),
> >>>>                cow_header.backing_file);
> >>>>
> >>>>  +    if (bs->backing_file[0] != '\0'&&  bdrv_is_fd_protocol(bs)) {
> >>>>  +        /* backing file currently not supported by fd: protocol */
> >>>>  +        goto fail;
> >>>>  +    }
> >> I don't think these checks are strictly needed. Without the check you
> >> can open the image itself using an fd, and the backing file using good
> >> old raw-posix. We shouldn't decide for our users that this isn't useful.
> >>
> >> In any case, it would have to move into block.c, you can't modify
> >> independent drivers for this.
> >>
> > 
> > I understand the point on not modifying independent drivers.
> > 
> > But if the backing file resides on NFS, wouldn't the the proposed 
> > SELinux changes prevent the open?
> 
> Probably. But what about cases where the backing file is local? Or even
> a non-libvirt, non-SELinux use case?
> 
> > Or are you talking about support where libvirt opens the backing file 
> > and passes the fd to Qemu?  If so there was some discussion about future 
> > support for this here: 
> > http://lists.gnu.org/archive/html/qemu-devel/2011-06/msg01496.html
> 
> Not really, but implementing this will be a bit easier if you don't
> forbid using backing files with fd.

In any case, for 'fd:' to be actually used by libvirt, we need to have
backing files supported. There are major users of libvirt who rely on
NFS and also use backing files, so an 'fd:' impl which can't deal with
the backing file problem isn't much use to libvirt.

Regards,
Daniel
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