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

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: [Xen-devel] RE: [RFC] tmem ABI change... backwards compatibility unnecessary?
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Thu, 2 Sep 2010 16:19:52 -0700 (PDT)
Cc: JeremyFitzhardinge <jeremy@xxxxxxxx>, "Xen-Devel \(xen-devel@xxxxxxxxxxxxxxxxxxx\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, tmem-devel@xxxxxxxxxxxxxx, Kurt Hackel <kurt.hackel@xxxxxxxxxx>, Vasiliy G Tolstov <v.tolstov@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, stephen.spector@xxxxxxxxxx, Chris Mason <chris.mason@xxxxxxxxxx>
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> From: Jan Beulich [mailto:JBeulich@xxxxxxxxxx]
> Sent: Wednesday, September 01, 2010 9:04 AM
> To: Dan Magenheimer
> Cc: stephen.spector@xxxxxxxxxx; Keir Fraser; JeremyFitzhardinge; Xen-
> Devel (xen-devel@xxxxxxxxxxxxxxxxxxx); Chris Mason; Kurt Hackel; tmem-
> devel@xxxxxxxxxxxxxx; Vasiliy G Tolstov
> Subject: Re: [RFC] tmem ABI change... backwards compatibility
> unnecessary?
> >>> On 01.09.10 at 16:36, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
> wrote:
> > I *think* it is still the case that tmem is experimental
> > and is not used anywhere yet in production.  If I am
> Well, if you call us shipping it (default disabled) in a couple of
> releases "not used in production"...
> > I am inclined to update the Xen tmem implementation
> > to only support v1 and gracefully fail v0.
> If "graceful" really means what it says, this would appear to be
> acceptable irrespective of my note above.

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!


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