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Re: [Qemu-devel] [PATCH v4 22/47] QEMU_VM_CMD_PACKAGED: Send a packaged


From: David Gibson
Subject: Re: [Qemu-devel] [PATCH v4 22/47] QEMU_VM_CMD_PACKAGED: Send a packaged chunk of migration stream
Date: Tue, 18 Nov 2014 15:36:43 +1100
User-agent: Mutt/1.5.23 (2014-03-12)

On Tue, Nov 04, 2014 at 10:19:15AM +0000, Dr. David Alan Gilbert wrote:
> * David Gibson (address@hidden) wrote:
> > On Fri, Oct 03, 2014 at 06:47:28PM +0100, Dr. David Alan Gilbert (git) 
> > wrote:
> > > From: "Dr. David Alan Gilbert" <address@hidden>
> > > 
> > > QEMU_VM_CMD_PACKAGED is a migration command that allows a chunk
> > > of migration stream to be sent in one go, and be received by
> > > a separate instance of the loadvm loop while not interacting
> > > with the migration stream.
> > > 
> > > This is used by postcopy to load device state (from the package)
> > > while loading memory pages from the main stream.
> > > 
> > > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > 
> > Reviewed-by: David Gibson <address@hidden>
> > 
> > Though one minor comment:
> > 
> > [snip]
> > > +/* We have a buffer of data to send; we don't want that all to be loaded
> > > + * by the command itself, so the command contains just the length of the
> > > + * extra buffer that we then send straight after it.
> > > + * TODO: Must be a better way to organise that
> > 
> > I'm not quite understanding what that comment's getting at.
> 
> We have these VM Commands; and they are a command type, and a length:
>      CMD_whatever
>      length: whatever
>      data for whatever
> 
> This comment is describing that, to make things easier for this code it's
> ended up as:
> 
>      CMD_PACKAGED
>      CMD length: 4    <--- i.e. just enough to hold the next 'length' field
>      package length
>     ---------------
>     The package
> 
> Which is a little different, hence i thought it needed the comment.

Ah.. right.  That seems.. gratuitously easy to get wrong further down
the track.  Why not just use the cmd length as the package length.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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