Re: [Qemu-devel] [PATCH v6 3/3] ivshmem: add check on protocol version in QEMU

[Stefan Hajnoczi]

On Mon, Sep 08, 2014 at 12:49:48PM +0300, Michael S. Tsirkin wrote:
> On Mon, Sep 08, 2014 at 11:17:50AM +0200, David Marchand wrote:
> > Send a protocol version as the first message from server, clients must close
> > communication if they don't support this protocol version.
> 
> What's the motivation here?
> This is at best a way to break all clients if an incompatible
> change in the server is made.
> Would not it be better to send a bitmap, or a list of supported
> versions, so it's possible to write servers compatible
> with multiple clients?

I'm not sure a full-fledged feature negotiation system is needed.  The
ivshmem protocol is local to the host and all participants are under
control of the administrator.

I suggested a protocol version to protect against misconfiguration.  For
example, building QEMU from source but talking to an outdated ivhsmem
server that is still running from before.

Remember that ivshmem-server and QEMU are shipped together by the
distro.  So in 99% of the cases they will have the same version anyway.
But we want to protect against rare misconfiguration that break things
(user mixing and matching incompatible software).

The only reason I can see for fancy negotiation is to make life easier
for proprietary third-party software, which I don't care about or like.

Stefan

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