Re: [Xen-devel] [Planning for Xen-4.6] Migration v2

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-11-25 at 19:54 +0000, Andrew Cooper wrote:
> There is an xl/libxl part of the migration v2 series which attempts to
> rectify this all in one go, as there is no alternative way of doing so. 
> The libxl section of the series is certainly not yet complete, but
> specific queries to the maintainers have thusfar gone unanswered.  On
> the other hand, the series does basically WorkForMe, including
> transparent legacy upgrade, suggesting that it is at least in an
> appropriate ballpark.

Is this, from "[PATCH 27/29] [VERY RFC] tools/libxl: Support restoring
legacy streams":

        This WorksForMe in the success case, but the error handling is 
certainly lacking.
        
        Specifically, the conversion scripts output fd can't be closed until 
the v2
        read loop has exited (cleanly or otherwise), without risking a 
close()/open()
        race silently replacing the fd behind the loops back.
        
        However, it can't be closed when the read loop exits, as the conversion 
script
        child might still be alive, and would prefer terminating cleaning than 
failing
        with a bad FD.
        
        Obviously, having one error handler block for the success/failure of 
the other
        side is a no-go, and would still involve a preselecting which was 
expected to
        exit first.
        
        Does anyone have any clever ideas of how to asynchronously collect the 
events
        "the conversion script has exited", "the save helper has exited" and 
"the v2
        read loop has finished" given the available infrastructure, to kick of a
        combined cleanup of all 3?

? I said then:
        
        This is probably one for Ian when he gets back, but a state machine
        which is cranked in response to the callbacks from the various
        completion events might be one way to approach this.
        
Prodding Ian again (by moving to the To: line...)

Was there any other questions? I've had a scrobble through the bit of v7
which 00/29 suggests might contain them, but that's the only one I saw.

Ian. 


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