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Re: [Xen-devel] [PATCH for-4.7] tools/libxl: Fix legacy migration following COLO backchannel breakage

On Thu, Apr 14, 2016 at 08:54:15PM +0100, Andrew Cooper wrote:
> c/s f5d947bf1b "tools/libxl: add back channel support to read stream"
> made a bogus adjustment to libxl__stream_read_start(), including
> removing the comment hinting at what was going on, which breaks
> conversion of a legacy migration stream.
> Symptoms look like:
>   root@anonymi:~ # xl migrate domU host
>   migration target: Ready to receive domain.
>   Saving to migration stream new xl format (info 0x1/0x0/2677)
>   xc: error: error polling suspend notification channel: -1: Internal error
>   Loading new save file <incoming migration stream> (new xl fmt info 
> 0x1/0x0/2677)
>    Savefile contains xl domain config in JSON format
>   Parsing config from <saved>
>   libxl: error: libxl_stream_read.c:327:stream_header_done: Invalid ident: 
> expected 0x4c6962786c466d74, got 0x01f00f0000000000
>   libxl: error: libxl_utils.c:430:libxl_read_exactly: file/stream truncated 
> reading ipc msg header from domain 1 save/restore helper stdout pipe
> The adjustment is not required for backchannel support (as there is no
> interaction between back channels and legacy conversion), and caused
> stream->fd to be latched in the datacopier before legacy conversion
> substitutes it for the fd which is the output of the conversion script.
> This causes libxl to consume data from the legacy stream rather than the
> v2 stream, and for the conversion script to encounter an error as the
> legacy stream appears to skip ahead.
> Undo the adjustments to libxl__stream_read_start(), and introduce a
> better description of what is going on.  Introduce some extra assertions
> to try and catch similar breakage in the future.
> Reported-by: Olaf Hering <olaf@xxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>

Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

Thank you for fixing this.

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