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

[Ian Campbell <Ian.Campbell@citrix.com>]

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>

> ---
> 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.