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

[Daniel P. Berrange]

On Wed, Jul 27, 2011 at 10:36:25AM +0200, Kevin Wolf wrote:
> Am 27.07.2011 10:22, schrieb Daniel P. Berrange:
> > 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.
> 
> I'm perfectly aware of that. But seriously, repeating it over and over
> again isn't going to make it happen any sooner. It requires -blockdev
> which we may or may not have by 1.0.

Yes, I understand we need also -blockdev for this. Other messages in this
mail thread have impied that this proposed patch on its own is useful for
libvirt's requirements, so I just wanted to remind people in general that
we can't use this patch on its own until we have something like -blockdev.

Regards,
Daniel
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