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[Xen-devel] Re: [RFC] tmem ABI change... backwards compatibility unneces

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: [Xen-devel] Re: [RFC] tmem ABI change... backwards compatibility unnecessary?
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Thu, 02 Sep 2010 16:39:08 -0700
Cc: Vasiliy G Tolstov <v.tolstov@xxxxxxxxx>, "Xen-Devel \(xen-devel@xxxxxxxxxxxxxxxxxxx\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, tmem-devel@xxxxxxxxxxxxxx, Kurt Hackel <kurt.hackel@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, stephen.spector@xxxxxxxxxx, Chris Mason <chris.mason@xxxxxxxxxx>
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 On 09/02/2010 04:19 PM, Dan Magenheimer wrote:
> OK, I will submit a patch tomorrow with the following characteristics:
> v0 (current) hypervisor + v0 guest: succeeds
> v1 (patched) hypervisor + v1 guest: succeeds
> v0 (current) hypervisor + v1 guest: fails
> v1 (patched) hypervisor + v0 guest: fails
> where fails is an xm dmesg message that says "unsupported
> spec version" when the guest attempts to create a pool.
> And pool creation failure ensures that all further tmem
> operations also fail (indeed never even result in a
> hypercall for most tmem-enabled kernels).
> Thank goodness ABI versioning was built into tmem from
> the beginning!

Hm, I'm not really a big fan of having a single "ABI version".  It
always seems better to have individual calls which can be
augmented/replaced by new calls, and/or have capability flags to extend
the ABI.  Versions mean you end up being stuck doing updates in a very
coarse-grained way, and the long-term support gets very onerous.
(Microsoft ABIs are a good antipattern to avoid, especially DirectX.)


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