Re: [PATCH v3 2/2] docs: update ivshmem device spec

[Eric Blake <eblake@redhat.com>]

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On 09/01/2014 03:52 AM, David Marchand wrote:

>>> What about upgrading QEMU and ivshmem-server while you have existing
>>> guests?  You cannot restart ivshmem-server, and the new QEMU would have
>>> to talk to the old ivshmem-server.
>>
>> Version negotiation also helps avoid confusion if someone combines
>> ivshmem-server and QEMU from different origins (e.g. built from source
>> and distro packaged).

Don't underestimate the likelihood of this happening.  Any long-running
process (which an ivshmem-server will be) continues running at the old
version, even when a package upgrade installs a new qemu binary; the new
binary should still be able to manage connections to the already-running
server.

Even neater would be a solution where an existing ivshmem-server could
re-exec an updated ivshmem-server binary that resulted from a distro
upgrade, hand over all state required for the new server to take over
from the point managed by the old server, so that you aren't stuck
running the old binary forever.  But that's a lot trickier to write, so
it is not necessary for a first implementation; and if you do that, then
you have the reverse situation to worry about (the new server must still
accept communication from existing old qemu binaries).

Note that the goal here is to support upgrades; it is probably okay if
downgrading from a new binary back to an old doesn't work correctly
(because the new software was using a feature not present in the old).

>>
>> It's a safeguard to prevent hard-to-diagnose failures when the system is
>> misconfigured.
>>
> 
> Hum, so you want the code to be defensive against mis-use, why not.
> 
> I wanted to keep modifications on ivshmem as little as possible in a
> first phase (all the more so as there are potential ivshmem users out
> there that I think will be impacted by a protocol change).

Existing ivshmem users MUST be aware that they are using something that
is not yet polished, and be prepared to make the upgrade to the polished
version.  It's best to minimize the hassle by making them upgrade
exactly once to a fully-robust version, rather than to have them upgrade
to a slightly-more robust version only to find out we didn't plan ahead
well enough to make further extensions in a back-compatible manner.

> 
> Sending the version as the first "vm_id" with an associated fd to -1
> before sending the real client id should work with existing QEMU client
> code (hw/misc/ivshmem.c).
> 
> Do you have a better idea ?
> Is there a best practice in QEMU for "version negotiation" that could
> work with ivshmem protocol ?

QMP starts off with a mandatory "qmp_capabilities" handshake, although
we haven't yet had to define any capabilities where cross-versioned
communication differs as a result.  Migration is somewhat of an example,
except that it is one-directional (we don't have a feedback path), so it
is somewhat best effort.  The qcow2 v3 file format is an example of
declaring features, rather than version numbers, and making decisions
about whether a feature is compatible (older clients can safely ignore
the bit, without corrupting the image but possibly having worse
performance) vs. incompatible (older clients must reject the image,
because not handling the feature correctly would corrupt the image).
The best handshakes are bi-directional - both sides advertise their
version (or better, their features), and a well-defined algorithm for
settling on the common subset of advertised features then ensures that
the two sides know how to talk to each other, or give a reason for
either side to disconnect early because of a missing feature.

> 
> I have a v4 ready with this (and all the pending comments), I will send
> it later unless a better idea is exposed.
> 
> 
> Thanks.
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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