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Re: [libvirt] [Qemu-devel] [PATCH v2] qemu-ga: add guest-sync-delimited



On 07.02.2012 02:09, Michael Roth wrote:
> guest-sync leaves it as an exercise to the user as to how to reliably
> obtain the response to guest-sync if the client had previously read in a
> partial response (due qemu-ga previously being restarted mid-"sentence"
> due to reboot, forced restart, etc).
> 
> qemu-ga handles this situation on its end by having a client precede
> their guest-sync request with a 0xFF byte (invalid UTF-8), which
> qemu-ga/QEMU JSON parsers will treat as a flush event. Thus we can
> reliably flush the qemu-ga parser state in preparation for receiving
> the guest-sync request.
> 
> guest-sync-delimited provides the same functionality for a client: when
> a guest-sync-delimited is issued, qemu-ga will precede it's response
> with a 0xFF byte that the client can use as an indicator to flush its
> buffer/parser state in preparation for reliably receiving the
> guest-sync-delimited response.
> 
> It is also useful as an optimization for clients, since, after issuing a
> guest-sync-delimited, clients can safely discard all stale data read
> from the channel until the 0xFF is found.
> 
> More information available on the wiki:
> 
> http://wiki.qemu.org/Features/QAPI/GuestAgent#QEMU_Guest_Agent_Protocol
> 
> Signed-off-by: Michael Roth <mdroth linux vnet ibm com>

This makes sense. And it's workable for libvirt.
IIUC, client can send 0xFF to the guest agent and vice versa, right?

Michal


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