Re: [PATCH for 4.5] libxl: Tell qemu to use raw format when using a tapdisk

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Dec 09, 2014 at 02:48:21PM +0000, Ian Campbell wrote:
> On Tue, 2014-12-09 at 14:04 +0000, George Dunlap wrote:
> > At the moment libxl unconditinally passes the underlying file format
> > to qemu in the device string.  However, when tapdisk is in use,
> > tapdisk handles the underlying format and presents qemu with
> > effectively a raw disk.  When qemu looks at the tapdisk block device
> > and doesn't find the image format it was looking for, it will fail.
> > 
> > This effectively means that tapdisk cannot be used with HVM domains at
> > the moment except for raw files.
> > 
> > Instead, if we're using a tapdisk backend, tell qemu to use a raw file
> > format.
> > 
> > Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
> 
> Acked-by: Ian Campbell <ian.campbell@citrix.com>

Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> 
> > ---
> > CC: Ian Campbell <ian.campbell@citrix.com>
> > CC: Ian Jackson <ian.jackson@citrix.com>
> > CC: Wei Liu <wei.liu2@citrix.com>
> > CC: Konrad Wilk <konrad.wilk@oracle.com>
> > 
> > Release exception justification:
> 
> I agree with your reasoning.
> 
> >  This fixes a bug in functionality, in
> > that at the moment HVM guests cannot boot with tapdisk and vhd format.
> > 
> > This is not a regression in xl functionality per se, since (AFAICT)
> > this has never worked.  However, given that 4.5 is the first release
> > without xend, this *does* represent a regression in functionality for
> > Xen as a whole (since before people using hvm guest with vhd on blktap
> > could use xend).
> > 
> > The fix is very simple and should only affect codepaths that already
> > don't work, so the risk of regressions should be very low.
> > 
> > While preparing this patch, I also noticed that cdroms will ignore the
> > backend parameter and treat everything as a file.  This is a bug but I
> > think it's a much less important one to address this late in the
> > release cycle.
> > ---
> >  tools/libxl/libxl_dm.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
> > index b25b574..10f3090 100644
> > --- a/tools/libxl/libxl_dm.c
> > +++ b/tools/libxl/libxl_dm.c
> > @@ -797,11 +797,14 @@ static char ** libxl__build_device_model_args_new(libxl__gc *gc,
> >                      continue;
> >                  }
> >  
> > -                if (disks[i].backend == LIBXL_DISK_BACKEND_TAP)
> > +                if (disks[i].backend == LIBXL_DISK_BACKEND_TAP) {
> > +                    format = qemu_disk_format_string(LIBXL_DISK_FORMAT_RAW);
> >                      pdev_path = libxl__blktap_devpath(gc, disks[i].pdev_path,
> >                                                        disks[i].format);
> > -                else
> > +                } else {
> >                      pdev_path = disks[i].pdev_path;
> > +                }
> > +
> >  
> >                  /*
> >                   * Explicit sd disks are passed through as is.
> 
>